module ActiveRecord
  class Migration
    class << self

      # Extend maintain_test_schema! to include migrations of the current wagon to test
      # or to make sure no wagon migrations are loaded when testing the main application.
      def maintain_test_schema_with_wagons!
        return unless maintain_test_schema?

        # set migrations paths to core only, wagon migrations are loaded separately
        Migrator.migrations_paths = Rails.application.paths['db/migrate'].to_a
        if app_needs_migration?
          suppress_messages { load_wagon_schema! }
        end
      end

      alias maintain_test_schema_without_wagons! maintain_test_schema!
      alias maintain_test_schema! maintain_test_schema_with_wagons!

      private

      def load_wagon_schema!
        Base.connection_handler.clear_all_connections!
        # Contrary to the original rails approach (#load_schema_if_pending!),
        # purge the database first to get rid of all wagon tables.
        config = Base.configurations.configs_for(env_name: 'test').first
        Tasks::DatabaseTasks.purge(config)

        Base.establish_connection(config)
        load_app_schema(config)

        Wagons.current_wagon.prepare_test_db if Wagons.current_wagon
      end

      def load_app_schema(config)
        Tasks::DatabaseTasks.load_schema(config)
        check_all_pending!
      end

      def app_needs_migration?
        Wagons.current_wagon ||
          defined_app_migration_versions != migration_versions_in_db
      end

      def defined_app_migration_versions
        migration_context.migrations.collect(&:version).to_set
      end

      def migration_versions_in_db
        if schema_migration_table_exists?
          migration_context.get_all_versions.to_set
        else
          [].to_set
        end
      end

      def schema_migration_table_exists?
        ActiveRecord::Base.connection.data_source_exists?(ActiveRecord::Base.schema_migrations_table_name)
      end

      def migration_context
        MigrationContext.new(Migrator.migrations_paths)
      end

      def maintain_test_schema?
        ActiveRecord.maintain_test_schema
      end

      def rails_version_smaller_than(version)
        Gem::Version.new(Rails::VERSION::STRING) < Gem::Version.new(version)
      end

    end
  end
end