DEV: Do one query per month when exporting chat messages (#22746)

We did some testing and saw that making one query per month is 
cheaper than querying all chat messages at ones. Note that even 
though the export job will be performing one query per month, 
the exported messages will be streamed into a single CSV file, so 
nothing changes from the user's point of view.

plugins/chat/lib/chat/messages_exporter.rb

@@ -4,10 +4,44 @@ module Chat
   module MessagesExporter
     LIMIT = 10_000
 
-    def chat_message_export
+    def chat_message_export(&block)
+      # perform 1 db query per month:
+      now = Time.now
+      from = 6.months.ago
+      while from <= now
+        export(from, from + 1.month, &block)
+        from = from + 1.month
+      end
+    end
+
+    def get_header(entity)
+      if entity === "chat_message"
+        %w[
+          id
+          chat_channel_id
+          chat_channel_name
+          user_id
+          username
+          message
+          cooked
+          created_at
+          updated_at
+          deleted_at
+          in_reply_to_id
+          last_editor_id
+          last_editor_username
+        ]
+      else
+        super
+      end
+    end
+
+    private
+
+    def export(from, to)
       Chat::Message
         .unscoped
-        .where(created_at: 6.months.ago..Time.current)
+        .where(created_at: from..to)
         .includes(:chat_channel)
         .includes(:user)
         .includes(:last_editor)
@@ -32,27 +66,5 @@ module Chat
           )
         end
     end
-
-    def get_header(entity)
-      if entity === "chat_message"
-        %w[
-          id
-          chat_channel_id
-          chat_channel_name
-          user_id
-          username
-          message
-          cooked
-          created_at
-          updated_at
-          deleted_at
-          in_reply_to_id
-          last_editor_id
-          last_editor_username
-        ]
-      else
-        super
-      end
-    end
   end
 end

plugins/chat/spec/system/admin/csv_exports_spec.rb

@@ -11,7 +11,10 @@ RSpec.describe "Chat CSV exports", type: :system do
   end
 
   xit "exports chat messages" do
-    message = Fabricate(:chat_message)
+    message_1 = Fabricate(:chat_message, created_at: 12.months.ago)
+    message_2 = Fabricate(:chat_message, created_at: 6.months.ago)
+    message_3 = Fabricate(:chat_message, created_at: 1.months.ago)
+    message_4 = Fabricate(:chat_message, created_at: Time.now)
 
     visit "/admin/plugins/chat"
     click_button "Create export"
@@ -39,8 +42,17 @@ RSpec.describe "Chat CSV exports", type: :system do
       ],
     )
 
+    assert_message(exported_data.second, message_1)
+    assert_message(exported_data.third, message_2)
+    assert_message(exported_data.fourth, message_3)
+    assert_message(exported_data.fifth, message_4)
+  ensure
+    csv_export_pm_page.clear_downloads
+  end
+
+  def assert_message(exported_message, message)
     time_format = "%Y-%m-%d %H:%M:%S UTC"
-    expect(exported_data.second).to eq(
+    expect(exported_message).to eq(
       [
         message.id.to_s,
         message.chat_channel.id.to_s,
@@ -57,7 +69,5 @@ RSpec.describe "Chat CSV exports", type: :system do
         message.last_editor.username,
       ],
     )
-  ensure
-    csv_export_pm_page.clear_downloads
   end
 end