FIX: from Sam: post_actions.targets_topic should not be nullable. Delete duplicate likes and prevent them from happening again.

db/migrate/20140815183851_fix_index_on_post_actions.rb

@@ -0,0 +1,26 @@
+class FixIndexOnPostActions < ActiveRecord::Migration
+  def change
+    execute 'UPDATE post_actions SET targets_topic = false WHERE targets_topic IS NULL'
+    change_column :post_actions, :targets_topic, :boolean, default: false, null: false
+
+    execute '
+    DELETE FROM post_actions pa
+                    USING post_actions x
+    WHERE pa.user_id = x.user_id AND
+          pa.post_action_type_id = x.post_action_type_id AND
+          pa.post_id = x.post_id AND
+          pa.targets_topic = x.targets_topic AND
+          pa.id < x.id AND
+          pa.deleted_at IS NULL AND
+          x.deleted_at IS NULL
+    '
+
+    remove_index "post_actions", name: "idx_unique_actions"
+    add_index "post_actions",
+                ["user_id", "post_action_type_id",
+                 "post_id", "targets_topic"],
+                name: "idx_unique_actions",
+                unique: true,
+                where: 'deleted_at IS NULL'
+  end
+end