diff --git a/framework/core/migrations/2018_01_11_093900_change_access_tokens_b8_columns.php b/framework/core/migrations/2018_01_11_093900_change_access_tokens_columns.php
similarity index 67%
rename from framework/core/migrations/2018_01_11_093900_change_access_tokens_b8_columns.php
rename to framework/core/migrations/2018_01_11_093900_change_access_tokens_columns.php
index 4e4965d3b..a27bad442 100644
--- a/framework/core/migrations/2018_01_11_093900_change_access_tokens_b8_columns.php
+++ b/framework/core/migrations/2018_01_11_093900_change_access_tokens_columns.php
@@ -19,19 +19,27 @@ return [
             $table->renameColumn('lifetime', 'lifetime_seconds');
             $table->renameColumn('last_activity', 'last_activity_at');
             $table->dateTime('created_at');
+            $table->integer('user_id')->unsigned()->change();
+        });
 
-            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+        // Use a separate schema instance because this column gets renamed
+        // in the first one.
+        $schema->table('access_tokens', function (Blueprint $table) {
+            $table->dateTime('last_activity_at')->change();
         });
     },
 
     'down' => function (Builder $schema) {
         $schema->table('access_tokens', function (Blueprint $table) {
+            $table->integer('last_activity_at')->change();
+        });
+
+        $schema->table('access_tokens', function (Blueprint $table) {
+            $table->renameColumn('token', 'id');
             $table->renameColumn('lifetime_seconds', 'lifetime');
             $table->renameColumn('last_activity_at', 'last_activity');
             $table->dropColumn('created_at');
-            $table->renameColumn('token', 'id');
-
-            $table->dropForeign(['user_id']);
+            $table->integer('user_id')->change();
         });
     }
 ];
diff --git a/framework/core/migrations/2018_01_11_094800_change_access_tokens_make_activity_at_column_datetime.php b/framework/core/migrations/2018_01_11_094000_change_access_tokens_add_foreign_keys.php
similarity index 53%
rename from framework/core/migrations/2018_01_11_094800_change_access_tokens_make_activity_at_column_datetime.php
rename to framework/core/migrations/2018_01_11_094000_change_access_tokens_add_foreign_keys.php
index 3e555f1ca..b6125506d 100644
--- a/framework/core/migrations/2018_01_11_094800_change_access_tokens_make_activity_at_column_datetime.php
+++ b/framework/core/migrations/2018_01_11_094000_change_access_tokens_add_foreign_keys.php
@@ -14,14 +14,23 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        // Delete rows with non-existent users so that we will be able to create
+        // foreign keys without any issues.
+        $schema->getConnection()
+            ->table('access_tokens')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->delete();
+
         $schema->table('access_tokens', function (Blueprint $table) {
-            $table->dateTime('last_activity_at')->change();
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
         });
     },
 
     'down' => function (Builder $schema) {
         $schema->table('access_tokens', function (Blueprint $table) {
-            $table->integer('last_activity_at')->change();
+            $table->dropForeign(['user_id']);
         });
     }
 ];
diff --git a/framework/core/migrations/2018_01_11_094900_change_api_keys_rename_id_to_key.php b/framework/core/migrations/2018_01_11_094900_change_api_keys_rename_id_to_key.php
deleted file mode 100644
index 7135025a7..000000000
--- a/framework/core/migrations/2018_01_11_094900_change_api_keys_rename_id_to_key.php
+++ /dev/null
@@ -1,29 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->table('api_keys', function (Blueprint $table) {
-            $table->renameColumn('id', 'key');
-            $table->dropPrimary(['id', 'key']);
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->table('api_keys', function (Blueprint $table) {
-            $table->renameColumn('key', 'id');
-            $table->primary('id');
-        });
-    }
-];
diff --git a/framework/core/migrations/2018_01_11_095000_change_api_keys_b8_columns.php b/framework/core/migrations/2018_01_11_095000_change_api_keys_columns.php
similarity index 73%
rename from framework/core/migrations/2018_01_11_095000_change_api_keys_b8_columns.php
rename to framework/core/migrations/2018_01_11_095000_change_api_keys_columns.php
index 478c578cf..eaa6246b4 100644
--- a/framework/core/migrations/2018_01_11_095000_change_api_keys_b8_columns.php
+++ b/framework/core/migrations/2018_01_11_095000_change_api_keys_columns.php
@@ -14,6 +14,12 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        $schema->table('api_keys', function (Blueprint $table) {
+            $table->dropPrimary(['id']);
+            $table->renameColumn('id', 'key');
+            $table->unique('key');
+        });
+
         $schema->table('api_keys', function (Blueprint $table) {
             $table->increments('id');
             $table->string('allowed_ips')->nullable();
@@ -31,5 +37,11 @@ return [
             $table->dropForeign(['user_id']);
             $table->dropColumn('id', 'allowed_ips', 'user_id', 'scopes', 'created_at');
         });
+
+        $schema->table('api_keys', function (Blueprint $table) {
+            $table->dropUnique(['key']);
+            $table->renameColumn('key', 'id');
+            $table->primary('id');
+        });
     }
 ];
diff --git a/framework/core/migrations/2018_01_11_102000_change_registration_tokens_rename_id_to_token.php b/framework/core/migrations/2018_01_11_102000_change_registration_tokens_rename_id_to_token.php
index 6a538622b..3a630513e 100644
--- a/framework/core/migrations/2018_01_11_102000_change_registration_tokens_rename_id_to_token.php
+++ b/framework/core/migrations/2018_01_11_102000_change_registration_tokens_rename_id_to_token.php
@@ -9,19 +9,6 @@
  * file that was distributed with this source code.
  */
 
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
+use Flarum\Database\Migration;
 
-return [
-    'up' => function (Builder $schema) {
-        $schema->table('registration_tokens', function (Blueprint $table) {
-            $table->renameColumn('id', 'token');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->table('registration_tokens', function (Blueprint $table) {
-            $table->renameColumn('token', 'id');
-        });
-    }
-];
+return Migration::renameColumn('registration_tokens', 'id', 'token');
diff --git a/framework/core/migrations/2018_01_11_155200_change_discussions_b8_columns.php b/framework/core/migrations/2018_01_11_155200_change_discussions_b8_columns.php
deleted file mode 100644
index 6485fdcab..000000000
--- a/framework/core/migrations/2018_01_11_155200_change_discussions_b8_columns.php
+++ /dev/null
@@ -1,56 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->table('discussions', function (Blueprint $table) {
-            $table->renameColumn('comments_count', 'comment_count');
-            $table->renameColumn('participants_count', 'participant_count');
-            $table->renameColumn('number_index', 'post_number_index');
-            $table->renameColumn('start_time', 'created_at');
-            $table->renameColumn('start_user_id', 'user_id');
-            $table->renameColumn('start_post_id', 'first_post_id');
-            $table->renameColumn('last_time', 'last_posted_at');
-            $table->renameColumn('last_user_id', 'last_posted_user_id');
-            $table->renameColumn('hide_time', 'hidden_at');
-            $table->renameColumn('hide_user_id', 'hidden_user_id');
-
-            $table->foreign('user_id')->references('id')->on('users')->onDelete('set null');
-            $table->foreign('last_posted_user_id')->references('id')->on('users')->onDelete('set null');
-            $table->foreign('hidden_user_id')->references('id')->on('users')->onDelete('set null');
-            $table->foreign('first_post_id')->references('id')->on('posts');
-            $table->foreign('last_post_id')->references('id')->on('posts');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->table('discussions', function (Blueprint $table) {
-            $table->renameColumn('comment_count', 'comments_count');
-            $table->renameColumn('participant_count', 'participants_count');
-            $table->renameColumn('post_number_index', 'number_index');
-            $table->renameColumn('created_at', 'start_time');
-            $table->renameColumn('user_id', 'start_user_id');
-            $table->renameColumn('first_post_id', 'start_post_id');
-            $table->renameColumn('last_posted_at', 'last_time');
-            $table->renameColumn('last_posted_user_id', 'last_user_id');
-            $table->renameColumn('hidden_at', 'hide_time');
-            $table->renameColumn('hidden_user_id', 'hide_user_id');
-
-            $table->dropForeign([
-                'user_id', 'last_posted_user_id', 'hidden_user_id',
-                'first_post_id', 'last_post_id'
-            ]);
-        });
-    }
-];
diff --git a/framework/core/migrations/2018_01_11_155200_change_discussions_rename_columns.php b/framework/core/migrations/2018_01_11_155200_change_discussions_rename_columns.php
new file mode 100644
index 000000000..3d4e7467d
--- /dev/null
+++ b/framework/core/migrations/2018_01_11_155200_change_discussions_rename_columns.php
@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::renameColumns('discussions', [
+    'comments_count' => 'comment_count',
+    'participants_count' => 'participant_count',
+    'number_index' => 'post_number_index',
+    'start_time' => 'created_at',
+    'start_user_id' => 'user_id',
+    'start_post_id' => 'first_post_id',
+    'last_time' => 'last_posted_at',
+    'last_user_id' => 'last_posted_user_id',
+    'hide_time' => 'hidden_at',
+    'hide_user_id' => 'hidden_user_id'
+]);
diff --git a/framework/core/migrations/2018_01_11_155300_change_discussions_add_foreign_keys.php b/framework/core/migrations/2018_01_11_155300_change_discussions_add_foreign_keys.php
new file mode 100644
index 000000000..6cf2adc44
--- /dev/null
+++ b/framework/core/migrations/2018_01_11_155300_change_discussions_add_foreign_keys.php
@@ -0,0 +1,53 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Illuminate\Database\Query\Expression;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        // Set non-existent entity IDs to NULL so that we will be able to create
+        // foreign keys without any issues.
+        $connection = $schema->getConnection();
+
+        $selectId = function ($table, $column) use ($connection) {
+            return new Expression(
+                '('.$connection->table($table)->whereRaw("id = $column")->select('id')->toSql().')'
+            );
+        };
+
+        $connection->table('discussions')->update([
+            'user_id' => $selectId('users', 'user_id'),
+            'last_posted_user_id' => $selectId('users', 'last_posted_user_id'),
+            'hidden_user_id' => $selectId('users', 'hidden_user_id'),
+            'first_post_id' => $selectId('posts', 'first_post_id'),
+            'last_post_id' => $selectId('posts', 'last_post_id'),
+        ]);
+
+        $schema->table('discussions', function (Blueprint $table) {
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('set null');
+            $table->foreign('last_posted_user_id')->references('id')->on('users')->onDelete('set null');
+            $table->foreign('hidden_user_id')->references('id')->on('users')->onDelete('set null');
+            $table->foreign('first_post_id')->references('id')->on('posts');
+            $table->foreign('last_post_id')->references('id')->on('posts');
+        });
+    },
+
+    'down' => function (Builder $schema) {
+        $schema->table('discussions', function (Blueprint $table) {
+            $table->dropForeign([
+                'user_id', 'last_posted_user_id', 'hidden_user_id',
+                'first_post_id', 'last_post_id'
+            ]);
+        });
+    }
+];
diff --git a/framework/core/migrations/2018_01_15_071700_change_user_discussions_discussions_users.php b/framework/core/migrations/2018_01_15_071700_rename_users_discussions_to_discussion_user.php
similarity index 77%
rename from framework/core/migrations/2018_01_15_071700_change_user_discussions_discussions_users.php
rename to framework/core/migrations/2018_01_15_071700_rename_users_discussions_to_discussion_user.php
index e1ac5e026..eaa51709d 100644
--- a/framework/core/migrations/2018_01_15_071700_change_user_discussions_discussions_users.php
+++ b/framework/core/migrations/2018_01_15_071700_rename_users_discussions_to_discussion_user.php
@@ -11,4 +11,4 @@
 
 use Flarum\Database\Migration;
 
-return Migration::renameTable('users_discussions', 'discussions_users');
+return Migration::renameTable('users_discussions', 'discussion_user');
diff --git a/framework/core/migrations/2018_01_15_071800_change_discussion_user_rename_columns.php b/framework/core/migrations/2018_01_15_071800_change_discussion_user_rename_columns.php
new file mode 100644
index 000000000..5fff7ea1e
--- /dev/null
+++ b/framework/core/migrations/2018_01_15_071800_change_discussion_user_rename_columns.php
@@ -0,0 +1,17 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::renameColumns('discussion_user', [
+    'read_time' => 'last_read_at',
+    'read_number' => 'last_read_post_number'
+]);
diff --git a/framework/core/migrations/2018_01_15_071800_change_discussions_users_b8_columns.php b/framework/core/migrations/2018_01_15_071800_change_discussions_users_b8_columns.php
deleted file mode 100644
index a0176f29d..000000000
--- a/framework/core/migrations/2018_01_15_071800_change_discussions_users_b8_columns.php
+++ /dev/null
@@ -1,34 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->table('discussions_users', function (Blueprint $table) {
-            $table->renameColumn('read_time', 'last_read_at');
-            $table->renameColumn('read_number', 'last_read_post_number');
-
-            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
-            $table->foreign('discussion_id')->references('id')->on('discussions')->onDelete('cascade');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->table('discussions_users', function (Blueprint $table) {
-            $table->renameColumn('last_read_at', 'read_time');
-            $table->renameColumn('last_read_post_number', 'read_number');
-
-            $table->dropForeign(['users_user_id', 'users_discussion_id']);
-        });
-    }
-];
diff --git a/framework/core/migrations/2018_01_15_071900_change_discussion_user_add_foreign_keys.php b/framework/core/migrations/2018_01_15_071900_change_discussion_user_add_foreign_keys.php
new file mode 100644
index 000000000..4a9d69a07
--- /dev/null
+++ b/framework/core/migrations/2018_01_15_071900_change_discussion_user_add_foreign_keys.php
@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        // Delete rows with non-existent entities so that we will be able to create
+        // foreign keys without any issues.
+        $connection = $schema->getConnection();
+        $connection->table('discussion_user')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->orWhereNotExists(function ($query) {
+                $query->selectRaw(1)->from('discussions')->whereRaw('id = discussion_id');
+            })
+            ->delete();
+
+        $schema->table('discussion_user', function (Blueprint $table) {
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+            $table->foreign('discussion_id')->references('id')->on('discussions')->onDelete('cascade');
+        });
+    },
+
+    'down' => function (Builder $schema) {
+        $schema->table('discussion_user', function (Blueprint $table) {
+            $table->dropForeign(['user_id', 'discussion_id']);
+        });
+    }
+];
diff --git a/framework/core/migrations/2018_01_15_072600_change_email_tokens_rename_id_to_token.php b/framework/core/migrations/2018_01_15_072600_change_email_tokens_rename_id_to_token.php
new file mode 100644
index 000000000..64586a4aa
--- /dev/null
+++ b/framework/core/migrations/2018_01_15_072600_change_email_tokens_rename_id_to_token.php
@@ -0,0 +1,14 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::renameColumn('email_tokens', 'id', 'token');
diff --git a/framework/core/migrations/2018_01_15_072600_change_email_tokens_b8_columns.php b/framework/core/migrations/2018_01_15_072700_change_email_tokens_add_foreign_keys.php
similarity index 66%
rename from framework/core/migrations/2018_01_15_072600_change_email_tokens_b8_columns.php
rename to framework/core/migrations/2018_01_15_072700_change_email_tokens_add_foreign_keys.php
index afcd3306f..c840045e6 100644
--- a/framework/core/migrations/2018_01_15_072600_change_email_tokens_b8_columns.php
+++ b/framework/core/migrations/2018_01_15_072700_change_email_tokens_add_foreign_keys.php
@@ -14,17 +14,22 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
-        $schema->table('email_tokens', function (Blueprint $table) {
-            $table->renameColumn('id', 'token');
+        // Delete rows with non-existent users so that we will be able to create
+        // foreign keys without any issues.
+        $schema->getConnection()
+            ->table('email_tokens')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->delete();
 
+        $schema->table('email_tokens', function (Blueprint $table) {
             $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
         });
     },
 
     'down' => function (Builder $schema) {
         $schema->table('email_tokens', function (Blueprint $table) {
-            $table->renameColumn('token', 'id');
-
             $table->dropForeign(['user_id']);
         });
     }
diff --git a/framework/core/migrations/2018_01_18_130400_rename_permissions_to_groups_permissions.php b/framework/core/migrations/2018_01_18_130400_rename_permissions_to_group_permission.php
similarity index 100%
rename from framework/core/migrations/2018_01_18_130400_rename_permissions_to_groups_permissions.php
rename to framework/core/migrations/2018_01_18_130400_rename_permissions_to_group_permission.php
diff --git a/framework/core/migrations/2018_07_19_101301_constraints_group_permission.php b/framework/core/migrations/2018_01_18_130500_change_group_permission_add_foreign_keys.php
similarity index 66%
rename from framework/core/migrations/2018_07_19_101301_constraints_group_permission.php
rename to framework/core/migrations/2018_01_18_130500_change_group_permission_add_foreign_keys.php
index 0f3092f64..f783547a6 100644
--- a/framework/core/migrations/2018_07_19_101301_constraints_group_permission.php
+++ b/framework/core/migrations/2018_01_18_130500_change_group_permission_add_foreign_keys.php
@@ -14,6 +14,15 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        // Delete rows with non-existent groups so that we will be able to create
+        // foreign keys without any issues.
+        $schema->getConnection()
+            ->table('group_permission')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('groups')->whereRaw('id = group_id');
+            })
+            ->delete();
+
         $schema->table('group_permission', function (Blueprint $table) {
             $table->foreign('group_id')->references('id')->on('groups')->onDelete('cascade');
         });
diff --git a/framework/core/migrations/2018_01_18_130400_rename_users_groups_to_group_user.php b/framework/core/migrations/2018_01_18_130600_rename_users_groups_to_group_user.php
similarity index 100%
rename from framework/core/migrations/2018_01_18_130400_rename_users_groups_to_group_user.php
rename to framework/core/migrations/2018_01_18_130600_rename_users_groups_to_group_user.php
diff --git a/framework/core/migrations/2018_07_19_101300_constraints_group_user.php b/framework/core/migrations/2018_01_18_130700_change_group_user_add_foreign_keys.php
similarity index 61%
rename from framework/core/migrations/2018_07_19_101300_constraints_group_user.php
rename to framework/core/migrations/2018_01_18_130700_change_group_user_add_foreign_keys.php
index 9f11a9f2a..028b35e94 100644
--- a/framework/core/migrations/2018_07_19_101300_constraints_group_user.php
+++ b/framework/core/migrations/2018_01_18_130700_change_group_user_add_foreign_keys.php
@@ -14,6 +14,18 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        // Delete rows with non-existent entities so that we will be able to create
+        // foreign keys without any issues.
+        $schema->getConnection()
+            ->table('group_user')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->orWhereNotExists(function ($query) {
+                $query->selectRaw(1)->from('groups')->whereRaw('id = group_id');
+            })
+            ->delete();
+
         $schema->table('group_user', function (Blueprint $table) {
             $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
             $table->foreign('group_id')->references('id')->on('groups')->onDelete('cascade');
diff --git a/framework/core/migrations/2018_01_18_132900_create_notifications_from_table.php b/framework/core/migrations/2018_01_18_132900_create_notifications_from_table.php
index 63fa38c8b..9ee23e2ed 100644
--- a/framework/core/migrations/2018_01_18_132900_create_notifications_from_table.php
+++ b/framework/core/migrations/2018_01_18_132900_create_notifications_from_table.php
@@ -9,21 +9,16 @@
  * file that was distributed with this source code.
  */
 
+use Flarum\Database\Migration;
 use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
 
-return [
-    'up' => function (Builder $schema) {
-        $schema->create('notifications_from', function (Blueprint $table) {
-            $table->integer('id')->unsigned();
-            $table->integer('from_user_id')->unsigned();
+return Migration::createTable(
+    'notifications_from',
+    function (Blueprint $table) {
+        $table->integer('id')->unsigned();
+        $table->integer('from_user_id')->unsigned();
 
-            $table->foreign('id')->references('id')->on('notifications')->onDelete('cascade');
-            $table->foreign('from_user_id')->references('id')->on('users')->onDelete('cascade');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->drop('notifications_from');
+        $table->foreign('id')->references('id')->on('notifications')->onDelete('cascade');
+        $table->foreign('from_user_id')->references('id')->on('users')->onDelete('cascade');
     }
-];
+);
diff --git a/framework/core/migrations/2018_01_18_132901_seed_notifications_from_table.php b/framework/core/migrations/2018_01_18_132901_seed_notifications_from_table.php
index c12fd4c22..c58ba7df2 100644
--- a/framework/core/migrations/2018_01_18_132901_seed_notifications_from_table.php
+++ b/framework/core/migrations/2018_01_18_132901_seed_notifications_from_table.php
@@ -13,16 +13,19 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
-        $schema->getConnection()->table('notifications')->chunkById(100, function ($notifications) use ($schema) {
-            foreach ($notifications as $notification) {
-                $insert = [
-                    'id' => $notification->id,
-                    'from_user_id' => $notification->sender_id
-                ];
+        $query = $schema->getConnection()->table('notifications')
+            ->whereExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = sender_id');
+            });
 
-                $schema->getConnection()->table('notifications_from')->updateOrInsert($insert, $insert);
-            }
-        });
+        foreach ($query->cursor() as $notification) {
+            $insert = [
+                'id' => $notification->id,
+                'from_user_id' => $notification->sender_id
+            ];
+
+            $schema->getConnection()->table('notifications_from')->updateOrInsert($insert, $insert);
+        }
     },
 
     'down' => function (Builder $schema) {
diff --git a/framework/core/migrations/2018_01_18_133000_change_notifications_b8_columns.php b/framework/core/migrations/2018_01_18_133000_change_notifications_columns.php
similarity index 93%
rename from framework/core/migrations/2018_01_18_133000_change_notifications_b8_columns.php
rename to framework/core/migrations/2018_01_18_133000_change_notifications_columns.php
index 01f2e34d6..963844b02 100644
--- a/framework/core/migrations/2018_01_18_133000_change_notifications_b8_columns.php
+++ b/framework/core/migrations/2018_01_18_133000_change_notifications_columns.php
@@ -21,8 +21,6 @@ return [
 
             $table->timestamp('read_at')->nullable();
             $table->timestamp('deleted_at')->nullable();
-
-            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
         });
 
         $schema->getConnection()->table('notifications')
@@ -48,13 +46,12 @@ return [
 
             $table->boolean('is_read');
             $table->boolean('is_deleted');
-
-            $table->dropForeign(['user_id']);
         });
 
         $schema->getConnection()->table('notifications')
             ->whereNotNull('read_at')
             ->update(['is_read' => 1]);
+
         $schema->getConnection()->table('notifications')
             ->whereNotNull('deleted_at')
             ->update(['is_deleted' => 1]);
diff --git a/framework/core/migrations/2018_01_18_133100_change_notifications_add_foreign_keys.php b/framework/core/migrations/2018_01_18_133100_change_notifications_add_foreign_keys.php
new file mode 100644
index 000000000..c71327d10
--- /dev/null
+++ b/framework/core/migrations/2018_01_18_133100_change_notifications_add_foreign_keys.php
@@ -0,0 +1,36 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        // Delete rows with non-existent users so that we will be able to create
+        // foreign keys without any issues.
+        $schema->getConnection()
+            ->table('notifications')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->delete();
+
+        $schema->table('notifications', function (Blueprint $table) {
+            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+        });
+    },
+
+    'down' => function (Builder $schema) {
+        $schema->table('notifications', function (Blueprint $table) {
+            $table->dropForeign(['user_id']);
+        });
+    }
+];
diff --git a/framework/core/migrations/2018_07_19_101301_constraints_password_tokens.php b/framework/core/migrations/2018_01_18_134500_change_password_tokens_add_foreign_keys.php
similarity index 65%
rename from framework/core/migrations/2018_07_19_101301_constraints_password_tokens.php
rename to framework/core/migrations/2018_01_18_134500_change_password_tokens_add_foreign_keys.php
index b5f499f87..7e3c3c6d4 100644
--- a/framework/core/migrations/2018_07_19_101301_constraints_password_tokens.php
+++ b/framework/core/migrations/2018_01_18_134500_change_password_tokens_add_foreign_keys.php
@@ -14,6 +14,15 @@ use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        // Delete rows with non-existent users so that we will be able to create
+        // foreign keys without any issues.
+        $connection = $schema->getConnection();
+        $connection->table('password_tokens')
+            ->whereNotExists(function ($query) {
+                $query->selectRaw(1)->from('users')->whereRaw('id = user_id');
+            })
+            ->delete();
+
         $schema->table('password_tokens', function (Blueprint $table) {
             $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
         });
diff --git a/framework/core/migrations/2018_01_18_135000_change_posts_rename_columns.php b/framework/core/migrations/2018_01_18_135000_change_posts_rename_columns.php
new file mode 100644
index 000000000..701c58aa1
--- /dev/null
+++ b/framework/core/migrations/2018_01_18_135000_change_posts_rename_columns.php
@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::renameColumns('posts', [
+    'time' => 'created_at',
+    'edit_time' => 'edited_at',
+    'hide_time' => 'hidden_at',
+    'edit_user_id' => 'edited_user_id',
+    'hide_user_id' => 'hidden_user_id'
+]);
diff --git a/framework/core/migrations/2018_01_18_135000_change_posts_b8_columns.php b/framework/core/migrations/2018_01_18_135100_change_posts_add_foreign_keys.php
similarity index 57%
rename from framework/core/migrations/2018_01_18_135000_change_posts_b8_columns.php
rename to framework/core/migrations/2018_01_18_135100_change_posts_add_foreign_keys.php
index bc7d57271..2a0623052 100644
--- a/framework/core/migrations/2018_01_18_135000_change_posts_b8_columns.php
+++ b/framework/core/migrations/2018_01_18_135100_change_posts_add_foreign_keys.php
@@ -9,21 +9,29 @@
  * file that was distributed with this source code.
  */
 
+use Illuminate\Database\Query\Expression;
 use Illuminate\Database\Schema\Blueprint;
 use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
+        // Set non-existent entity IDs to NULL so that we will be able to create
+        // foreign keys without any issues.
+        $connection = $schema->getConnection();
+
+        $selectId = function ($table, $column) use ($connection) {
+            return new Expression(
+                '('.$connection->table($table)->whereRaw("id = $column")->select('id')->toSql().')'
+            );
+        };
+
+        $connection->table('posts')->update([
+            'user_id' => $selectId('users', 'user_id'),
+            'edited_user_id' => $selectId('users', 'edited_user_id'),
+            'hidden_user_id' => $selectId('users', 'hidden_user_id'),
+        ]);
+
         $schema->table('posts', function (Blueprint $table) {
-            $table->renameColumn('time', 'created_at');
-            $table->renameColumn('edit_time', 'edited_at');
-            $table->renameColumn('hide_time', 'hidden_at');
-
-            $table->renameColumn('edit_user_id', 'edited_user_id');
-            $table->renameColumn('hide_user_id', 'hidden_user_id');
-
-            $table->longText('content')->change();
-
             $table->foreign('user_id')->references('id')->on('users')->onDelete('set null');
             $table->foreign('edited_user_id')->references('id')->on('users')->onDelete('set null');
             $table->foreign('hidden_user_id')->references('id')->on('users')->onDelete('set null');
@@ -32,15 +40,6 @@ return [
 
     'down' => function (Builder $schema) {
         $schema->table('posts', function (Blueprint $table) {
-            $table->renameColumn('created_at', 'time');
-            $table->renameColumn('edited_at', 'edit_time');
-            $table->renameColumn('hidden_at', 'hide_time');
-
-            $table->renameColumn('edited_user_id', 'edit_user_id');
-            $table->renameColumn('edited_user_id', 'hidden_user_id');
-
-            $table->mediumText('content')->change();
-
             $table->dropForeign([
                 'user_id', 'discussion_id',
                 'edited_user_id', 'hidden_user_id'
diff --git a/framework/core/migrations/2018_01_30_220100_create_post_user_table.php b/framework/core/migrations/2018_01_30_220100_create_post_user_table.php
new file mode 100644
index 000000000..577ce3511
--- /dev/null
+++ b/framework/core/migrations/2018_01_30_220100_create_post_user_table.php
@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+return Migration::createTable(
+    'post_user',
+    function (Blueprint $table) {
+        $table->integer('post_id')->unsigned();
+        $table->integer('user_id')->unsigned();
+
+        $table->primary(['post_id', 'user_id']);
+
+        $table->foreign('post_id')->references('id')->on('posts')->onDelete('cascade');
+        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+    }
+);
diff --git a/framework/core/migrations/2018_01_30_220100_create_posts_users_table.php b/framework/core/migrations/2018_01_30_220100_create_posts_users_table.php
deleted file mode 100644
index ba1d3e952..000000000
--- a/framework/core/migrations/2018_01_30_220100_create_posts_users_table.php
+++ /dev/null
@@ -1,29 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->create('posts_users', function (Blueprint $table) {
-            $table->integer('post_id')->unsigned();
-            $table->integer('user_id')->unsigned();
-
-            $table->foreign('post_id')->references('id')->on('posts')->onDelete('cascade');
-            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->drop('posts_users');
-    }
-];
diff --git a/framework/core/migrations/2018_01_30_222900_change_users_b8_columns.php b/framework/core/migrations/2018_01_30_222900_change_users_b8_columns.php
deleted file mode 100644
index e7bb77c9e..000000000
--- a/framework/core/migrations/2018_01_30_222900_change_users_b8_columns.php
+++ /dev/null
@@ -1,41 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->table('users', function (Blueprint $table) {
-            $table->renameColumn('is_activated', 'is_email_confirmed');
-            $table->renameColumn('join_time', 'joined_at');
-            $table->renameColumn('last_seen_time', 'last_seen_at');
-            $table->renameColumn('discussions_count', 'discussion_count');
-            $table->renameColumn('comments_count', 'comment_count');
-            $table->renameColumn('read_time', 'marked_all_as_read_at');
-            $table->renameColumn('notifications_read_time', 'read_notifications_at');
-            $table->renameColumn('avatar_path', 'avatar_url');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->table('users', function (Blueprint $table) {
-            $table->renameColumn('is_email_confirmed', 'is_activated');
-            $table->renameColumn('joined_at', 'join_time');
-            $table->renameColumn('last_seen_at', 'last_seen_time');
-            $table->renameColumn('discussion_count', 'discussions_count');
-            $table->renameColumn('comment_count', 'comments_count');
-            $table->renameColumn('marked_all_as_read_at', 'read_time');
-            $table->renameColumn('read_notifications_at', 'notifications_read_time');
-            $table->renameColumn('avatar_url', 'avatar_path');
-        });
-    }
-];
diff --git a/framework/core/migrations/2018_01_30_222900_change_users_rename_columns.php b/framework/core/migrations/2018_01_30_222900_change_users_rename_columns.php
new file mode 100644
index 000000000..4b6965bde
--- /dev/null
+++ b/framework/core/migrations/2018_01_30_222900_change_users_rename_columns.php
@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::renameColumns('users', [
+    'is_activated' => 'is_email_confirmed',
+    'join_time' => 'joined_at',
+    'last_seen_time' => 'last_seen_at',
+    'discussions_count' => 'discussion_count',
+    'comments_count' => 'comment_count',
+    'read_time' => 'marked_all_as_read_at',
+    'notifications_read_time' => 'read_notifications_at',
+    'avatar_path' => 'avatar_url'
+]);
diff --git a/framework/core/migrations/2018_01_30_223700_create_user_user_table.php b/framework/core/migrations/2018_01_30_223700_create_user_user_table.php
new file mode 100644
index 000000000..60c845ff6
--- /dev/null
+++ b/framework/core/migrations/2018_01_30_223700_create_user_user_table.php
@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+use Illuminate\Database\Schema\Blueprint;
+
+return Migration::createTable(
+    'user_user',
+    function (Blueprint $table) {
+        $table->integer('user_id')->unsigned();
+        $table->integer('other_user_id')->unsigned();
+
+        $table->foreign('user_id')->references('id')->on('posts')->onDelete('cascade');
+        $table->foreign('other_user_id')->references('id')->on('users')->onDelete('cascade');
+    }
+);
diff --git a/framework/core/migrations/2018_01_30_223700_create_users_users_table.php b/framework/core/migrations/2018_01_30_223700_create_users_users_table.php
deleted file mode 100644
index ed74651c1..000000000
--- a/framework/core/migrations/2018_01_30_223700_create_users_users_table.php
+++ /dev/null
@@ -1,29 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Illuminate\Database\Schema\Blueprint;
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        $schema->create('users_users', function (Blueprint $table) {
-            $table->integer('user_id')->unsigned();
-            $table->integer('other_user_id')->unsigned();
-
-            $table->foreign('user_id')->references('id')->on('posts')->onDelete('cascade');
-            $table->foreign('other_user_id')->references('id')->on('users')->onDelete('cascade');
-        });
-    },
-
-    'down' => function (Builder $schema) {
-        $schema->drop('users_users');
-    }
-];
diff --git a/framework/core/src/Database/Migration.php b/framework/core/src/Database/Migration.php
index 79332813d..6469b847f 100644
--- a/framework/core/src/Database/Migration.php
+++ b/framework/core/src/Database/Migration.php
@@ -78,16 +78,28 @@ abstract class Migration
      * Rename a column.
      */
     public static function renameColumn($tableName, $from, $to)
+    {
+        return static::renameColumns($tableName, [$from => $to]);
+    }
+
+    /**
+     * Rename multiple columns.
+     */
+    public static function renameColumns($tableName, array $columnNames)
     {
         return [
-            'up' => function (Builder $schema) use ($tableName, $from, $to) {
-                $schema->table($tableName, function (Blueprint $table) use ($from, $to) {
-                    $table->renameColumn($from, $to);
+            'up' => function (Builder $schema) use ($tableName, $columnNames) {
+                $schema->table($tableName, function (Blueprint $table) use ($columnNames) {
+                    foreach ($columnNames as $from => $to) {
+                        $table->renameColumn($from, $to);
+                    }
                 });
             },
-            'down' => function (Builder $schema) use ($tableName, $from, $to) {
-                $schema->table($tableName, function (Blueprint $table) use ($from, $to) {
-                    $table->renameColumn($to, $from);
+            'down' => function (Builder $schema) use ($tableName, $columnNames) {
+                $schema->table($tableName, function (Blueprint $table) use ($columnNames) {
+                    foreach ($columnNames as $to => $from) {
+                        $table->renameColumn($from, $to);
+                    }
                 });
             }
         ];