Added util class to support squashing migrations

db/migrate/migration_utils/migration_squasher.rb

@@ -0,0 +1,82 @@
+# 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
+