Use batching to clear orphans in head_pipeline migration

db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb

@@ -5,15 +5,24 @@ class AddForeignKeyToMergeRequests < ActiveRecord::Migration
 
   disable_ddl_transaction!
 
+  class MergeRequest < ActiveRecord::Base
+    self.table_name = 'merge_requests'
+    include ::EachBatch
+  end
+
   def up
-    execute <<-SQL.strip_heredoc
-      UPDATE merge_requests SET head_pipeline_id = null
-        WHERE NOT EXISTS (
+    scope = <<-SQL.strip_heredoc
+      head_pipeline_id IS NOT NULL
+        AND NOT EXISTS (
           SELECT 1 FROM ci_pipelines
             WHERE ci_pipelines.id = merge_requests.head_pipeline_id
         )
     SQL
 
+    MergeRequest.where(scope).each_batch(of: 1000) do |merge_requests|
+      merge_requests.update_all(head_pipeline_id: nil)
+    end
+
     unless foreign_key_exists?(:merge_requests, :head_pipeline_id)
       add_concurrent_foreign_key(:merge_requests, :ci_pipelines,
                                  column: :head_pipeline_id, on_delete: :nullify)

spec/migrations/add_foreign_key_to_merge_requests_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20170713104829_add_foreign_key_to_merge_requests.rb')
+
+describe AddForeignKeyToMergeRequests, :migration do
+  let(:projects) { table(:projects) }
+  let(:merge_requests) { table(:merge_requests) }
+  let(:pipelines) { table(:ci_pipelines) }
+
+  before do
+    projects.create!(name: 'gitlab', path: 'gitlab-org/gitlab-ce')
+    pipelines.create!(project_id: projects.first.id,
+                      ref: 'some-branch',
+                      sha: 'abc12345')
+
+    # merge request without a pipeline
+    create_merge_request(head_pipeline_id: nil)
+
+    # merge request with non-existent pipeline
+    create_merge_request(head_pipeline_id: 1234)
+
+    # merge reqeust with existing pipeline assigned
+    create_merge_request(head_pipeline_id: pipelines.first.id)
+  end
+
+  it 'correctly adds a foreign key to head_pipeline_id' do
+    migrate!
+
+    expect(merge_requests.first.head_pipeline_id).to be_nil
+    expect(merge_requests.second.head_pipeline_id).to be_nil
+    expect(merge_requests.third.head_pipeline_id).to eq pipelines.first.id
+  end
+
+  def create_merge_request(**opts)
+    merge_requests.create!(source_project_id: projects.first.id,
+                           target_project_id: projects.first.id,
+                           source_branch: 'some-branch',
+                           target_branch: 'master', **opts)
+  end
+end