Add data migration to update draft on merge_requests

Populate the field as "true" for all open MRs whose titles would
currently match `#draft?`

Changelog: other

db/post_migrate/20210526222715_backfill_draft_status_on_merge_requests.rb

+# frozen_string_literal: true
+
+class BackfillDraftStatusOnMergeRequests < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  INDEX_NAME = "tmp_index_merge_requests_draft_and_status"
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, :id,
+      where: "draft = false AND state_id = 1 AND ((title)::text ~* '^\\[draft\\]|\\(draft\\)|draft:|draft|\\[WIP\\]|WIP:|WIP'::text)",
+      name: INDEX_NAME
+
+    update_column_in_batches(:merge_requests, :draft, true, batch_size: 100) do |table, query|
+      query
+        .where(table[:state_id].eq(1))
+        .where(table[:draft].eq(false))
+        .where(table[:title].matches_regexp('^\\[draft\\]|\\(draft\\)|draft:|draft|\\[WIP\\]|WIP:|WIP', false))
+    end
+
+    remove_concurrent_index_by_name :merge_requests, INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :merge_requests, INDEX_NAME
+  end
+end

db/schema_migrations/20210526222715

+f80787d85538cedaba34cb204c98df2d0bbbf85f438d4df8f1187d2f4d881588
\ No newline at end of file

spec/migrations/backfill_draft_status_on_merge_requests_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20210526222715_backfill_draft_status_on_merge_requests.rb')
+
+RSpec.describe BackfillDraftStatusOnMergeRequests, :migration do
+  let(:namespaces)     { table(:namespaces) }
+  let(:projects)       { table(:projects) }
+  let(:merge_requests) { table(:merge_requests) }
+
+  let(:group)   { namespaces.create!(name: 'gitlab', path: 'gitlab') }
+  let(:project) { projects.create!(namespace_id: group.id) }
+
+  let(:draft_prefixes) { ["[Draft]", "(Draft)", "Draft:", "Draft", "[WIP]", "WIP:", "WIP"] }
+
+  def create_merge_request(params)
+    common_params = {
+      target_project_id: project.id,
+      target_branch: 'feature1',
+      source_branch: 'master'
+    }
+
+    merge_requests.create!(common_params.merge(params))
+  end
+
+  context "for MRs with #draft? == true titles but draft attribute false" do
+    before do
+      draft_prefixes.each do |prefix|
+        (1..4).each do |n|
+          merge_request = create_merge_request(title: "#{prefix} This is a title", state_id: n)
+          merge_request.update_columns(draft: false)
+        end
+      end
+    end
+
+    it "updates all open draft merge request's draft field to true" do
+      mr_count = merge_requests.all.count
+
+      expect { disable_migrations_output { migrate! } }
+        .to change { MergeRequest.where(draft: false).count }
+        .from(mr_count).to(mr_count - draft_prefixes.length)
+    end
+  end
+end