1 files changed, 45 insertions, 0 deletions
diff --git a/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb b/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb
new file mode 100644
index 00000000000..c25d4fd5986
--- /dev/null
+++ b/db/migrate/20170713104829_add_foreign_key_to_merge_requests.rb
@@ -0,0 +1,45 @@
+class AddForeignKeyToMergeRequests < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class MergeRequest < ActiveRecord::Base
+    self.table_name = 'merge_requests'
+    include ::EachBatch
+  end
+
+  def up
+    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)
+    end
+  end
+
+  def down
+    if foreign_key_exists?(:merge_requests, :head_pipeline_id)
+      remove_foreign_key(:merge_requests, column: :head_pipeline_id)
+    end
+  end
+
+  private
+
+  def foreign_key_exists?(table, column)
+    foreign_keys(table).any? do |key|
+      key.options[:column] == column.to_s
+    end
+  end
+end