# This migration aggregates the migrations passed in migrations into one given as a block
# heredoc
module Migration
  class MigrationSquasher
    class IncompleteMigrationsError < ::StandardError
    end

    #define all the following methods as class methods
    class << self

      def squash(aggregated_versions)
        intersection = aggregated_versions & all_versions

        if intersection == []

          # No migrations that this migration aggregates have already been
          # applied. In this case, run the aggregated migration passed as a block
          yield

        elsif intersection == aggregated_versions

          # All migrations that this migration aggregates have already
          # been applied. In this case, remove the information about those
          # migrations from the schema_migrations table and we're done.
          ActiveRecord::Base.connection.execute <<-SQL + (intersection.map { |version| <<-CONDITIONS }).join(" OR ")
            DELETE FROM
              #{quoted_schema_migrations_table_name}
            WHERE
          SQL
            #{version_column_for_comparison} = #{quote_value(version)}
          CONDITIONS

        else

          missing = aggregated_versions - intersection

          # Only a part of the migrations that this migration aggregates
          # have already been applied. In this case, fail miserably.
          raise IncompleteMigrationsError, <<-MESSAGE.split("\n").map(&:strip!).join(" ") + "\n"
            It appears you are migrating from an incompatible version.
            Your database has only some migrations to be squashed.
            Please update your installation to a version including all the
            aggregated migrations and run this migration again.
            The following migrations are missing: #{missing}
          MESSAGE

        end

      end


    private

      def all_versions
        table = Arel::Table.new(schema_migrations_table_name)
        ActiveRecord::Base.connection.select_values(table.project(table['version']))
      end

      def schema_migrations_table_name
        ActiveRecord::Migrator.schema_migrations_table_name
      end

      def quoted_schema_migrations_table_name
        ActiveRecord::Base.connection.quote_table_name(schema_migrations_table_name)
      end

      def quoted_version_column_name
        ActiveRecord::Base.connection.quote_table_name("version")
      end

      def version_column_for_comparison
        "#{quoted_schema_migrations_table_name}.#{quoted_version_column_name}"
      end

      def quote_value s
        ActiveRecord::Base.connection.quote(s)
      end

    end
  end
end