3 files changed, 47 insertions, 2 deletions
diff --git a/rubocop/cop/migration/update_column_in_batches.rb b/rubocop/cop/migration/update_column_in_batches.rb
new file mode 100644
index 00000000000..3f886cbfea3
--- /dev/null
+++ b/rubocop/cop/migration/update_column_in_batches.rb
@@ -0,0 +1,43 @@
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if a spec file exists for any migration using
+      # `update_column_in_batches`.
+      class UpdateColumnInBatches < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'Migration running `update_column_in_batches` must have a spec file at' \
+          ' `%s`.'.freeze
+
+        def on_send(node)
+          return unless in_migration?(node)
+          return unless node.children[1] == :update_column_in_batches
+
+          spec_path = spec_filename(node)
+
+          unless File.exist?(File.expand_path(spec_path, rails_root))
+            add_offense(node, :expression, format(MSG, spec_path))
+          end
+        end
+
+        private
+
+        def spec_filename(node)
+          source_name = node.location.expression.source_buffer.name
+          path = Pathname.new(source_name).relative_path_from(rails_root)
+          dirname = File.dirname(path)
+            .sub(%r{\Adb/(migrate|post_migrate)}, 'spec/migrations')
+          filename = File.basename(source_name, '.rb').sub(%r{\A\d+_}, '')
+
+          File.join(dirname, "#{filename}_spec.rb")
+        end
+
+        def rails_root
+          Pathname.new(File.expand_path('../../..', __dir__))
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/migration_helpers.rb b/rubocop/migration_helpers.rb
index 3160a784a04..c3473771178 100644
--- a/rubocop/migration_helpers.rb
+++ b/rubocop/migration_helpers.rb
@@ -3,8 +3,9 @@ module RuboCop
   module MigrationHelpers
     # Returns true if the given node originated from the db/migrate directory.
     def in_migration?(node)
-      File.dirname(node.location.expression.source_buffer.name).
-        end_with?('db/migrate')
+      dirname = File.dirname(node.location.expression.source_buffer.name)
+
+      dirname.end_with?('db/migrate', 'db/post_migrate')
     end
   end
 end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 4ff204f939e..b65efbc41f4 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -8,3 +8,4 @@ require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'
+require_relative 'cop/migration/update_column_in_batches'