1 files changed, 31 insertions, 0 deletions
diff --git a/tooling/danger/change_column_default.rb b/tooling/danger/change_column_default.rb
new file mode 100644
index 00000000000..57ebb06a656
--- /dev/null
+++ b/tooling/danger/change_column_default.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require_relative 'suggestor'
+
+module Tooling
+  module Danger
+    module ChangeColumnDefault
+      include ::Tooling::Danger::Suggestor
+
+      METHODS = %w[change_column_default remove_column_default].freeze
+      MIGRATION_METHODS_REGEX = /^\+\s*(.*\.)?(#{METHODS.join('|')})[(\s]/
+      MIGRATION_FILES_REGEX = %r{^db/(post_)?migrate}
+
+      DOCUMENTATION = 'https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#changing-column-defaults'
+      COMMENT =
+        "Changing column default is difficult because of how Rails handles values that are equal to the default. " \
+        "Please make sure all columns are declared as `columns_changing_default`. " \
+        "For more information, see [Avoiding downtime in migrations documentation](#{DOCUMENTATION}).".freeze
+
+      def add_comment_for_change_column_default
+        migration_files.each do |filename|
+          add_suggestion(filename: filename, regex: MIGRATION_METHODS_REGEX, comment_text: COMMENT)
+        end
+      end
+
+      def migration_files
+        helper.all_changed_files.grep(MIGRATION_FILES_REGEX)
+      end
+    end
+  end
+end