Clean up `noteable_id` for notes on commits

This was incorrectly set by a bug in:
https://gitlab.com/gitlab-org/gitlab-ce/issues/54924

db/migrate/20190228092516_clean_up_noteable_id_for_notes_on_commits.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class CleanUpNoteableIdForNotesOnCommits < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  TEMP_INDEX_NAME = 'index_notes_on_commit_with_null_noteable_id'
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index_by_name(:notes, TEMP_INDEX_NAME)
+
+    add_concurrent_index(:notes, :id, where: "noteable_type = 'Commit' AND noteable_id IS NOT NULL", name: TEMP_INDEX_NAME)
+
+    # rubocop:disable Migration/UpdateLargeTable
+    update_column_in_batches(:notes, :noteable_id, nil) do |table, query|
+      query.where(
+        table[:noteable_type].eq('Commit').and(table[:noteable_id].not_eq(nil))
+      )
+    end
+
+    remove_concurrent_index_by_name(:notes, TEMP_INDEX_NAME)
+  end
+
+  def down
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20190220150130) do
+ActiveRecord::Schema.define(version: 20190228092516) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

spec/migrations/clean_up_noteable_id_for_notes_on_commits_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20190228092516_clean_up_noteable_id_for_notes_on_commits.rb')
+
+describe CleanUpNoteableIdForNotesOnCommits, :migration do
+  let(:notes) { table(:notes) }
+
+  before do
+    notes.create!(noteable_type: 'Commit', commit_id: '3d0a182204cece4857f81c6462720e0ad1af39c9', noteable_id: 3, note: 'Test')
+    notes.create!(noteable_type: 'Commit', commit_id: '3d0a182204cece4857f81c6462720e0ad1af39c9', noteable_id: 3, note: 'Test')
+    notes.create!(noteable_type: 'Commit', commit_id: '3d0a182204cece4857f81c6462720e0ad1af39c9', noteable_id: 3, note: 'Test')
+
+    notes.create!(noteable_type: 'Issue', noteable_id: 1, note: 'Test')
+    notes.create!(noteable_type: 'MergeRequest', noteable_id: 1, note: 'Test')
+    notes.create!(noteable_type: 'Snippet', noteable_id: 1, note: 'Test')
+  end
+
+  it 'clears noteable_id for notes on commits' do
+    expect { migrate! }.to change { dirty_notes_on_commits.count }.from(3).to(0)
+  end
+
+  it 'does not clear noteable_id for other notes' do
+    expect { migrate! }.not_to change { other_notes.count }
+  end
+
+  def dirty_notes_on_commits
+    notes.where(noteable_type: 'Commit').where('noteable_id IS NOT NULL')
+  end
+
+  def other_notes
+    notes.where("noteable_type != 'Commit' AND noteable_id IS NOT NULL")
+  end
+end