Patch MySQL's odd foreign key behavior.

db/migrate/20180320182229_add_indexes_for_user_activity_queries.rb

@@ -12,6 +12,19 @@ class AddIndexesForUserActivityQueries < ActiveRecord::Migration
 
   def down
     remove_concurrent_index :events, [:project_id, :author_id]
+
+    patch_foreign_keys do
       remove_concurrent_index :user_interacted_projects, :user_id
     end
+  end
+
+  private
+
+  def patch_foreign_keys
+    # MySQL doesn't like to remove the index with a foreign key using it.
+    remove_foreign_key :user_interacted_projects, :users unless Gitlab::Database.postgresql?
+    yield
+    # Let's re-add the foreign key using the existing index on (user_id, project_id)
+    add_concurrent_foreign_key :user_interacted_projects, :users, column: :user_id unless Gitlab::Database.postgresql?
+  end
 end