FIX: Prevent field type migration from poisoning AR cache (#27549)

We previously migrated field_type from a string to an integer backed enum. Part of this involved renaming a column in a post migration, swapping out field_type:string for field_type:integer. This borks the ActiveRecord cache since the application is already running. Rebooting fixes it, but we want to avoid having this happen in the first place.

db/migrate/20240620024938_add_back_field_type_enum_to_user_fields.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class AddBackFieldTypeEnumToUserFields < ActiveRecord::Migration[7.0]
+  def change
+    # NOTE: This is here to undo the swap done in SwapFieldTypeWithFieldTypeEnumOnUserFields,
+    #       as that change was breaking the AR cache until the application is rebooted.
+    #       The condition here is to ensure it's only executed if that post-migration has been
+    #       applied.
+    if !ActiveRecord::Base.connection.column_exists?(:user_fields, :field_type_enum)
+      add_column :user_fields, :field_type_enum, :integer
+      change_column_null :user_fields, :field_type, true
+
+      execute(<<~SQL)
+        UPDATE user_fields
+        SET field_type_enum = field_type
+      SQL
+
+      change_column_null :user_fields, :field_type_enum, false
+    end
+  end
+end

db/post_migrate/20240612073116_swap_field_type_with_field_type_enum_on_user_fields.rb

@@ -1,14 +1,17 @@
 # frozen_string_literal: true
 
 class SwapFieldTypeWithFieldTypeEnumOnUserFields < ActiveRecord::Migration[7.0]
-  DROPPED_COLUMNS ||= { user_fields: %i[field_type] }
+  # DROPPED_COLUMNS ||= { user_fields: %i[field_type] }
 
-  def up
-    DROPPED_COLUMNS.each { |table, columns| Migration::ColumnDropper.execute_drop(table, columns) }
-    rename_column :user_fields, :field_type_enum, :field_type
-  end
+  # def up
+  #   # WARNING: Swapping in a column of a different type in a post-migration will break the AR
+  #   #          cache, since the application is already booted, requiring a restart.
+  #   #
+  #   DROPPED_COLUMNS.each { |table, columns| Migration::ColumnDropper.execute_drop(table, columns) }
+  #   rename_column :user_fields, :field_type_enum, :field_type
+  # end
 
-  def down
-    raise ActiveRecord::IrreversibleMigration
-  end
+  # def down
+  #   raise ActiveRecord::IrreversibleMigration
+  # end
 end