1 files changed, 66 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/attribute_decorators.rb b/activerecord/lib/active_record/attribute_decorators.rb
new file mode 100644
index 0000000000..5b96623b6e
--- /dev/null
+++ b/activerecord/lib/active_record/attribute_decorators.rb
@@ -0,0 +1,66 @@
+module ActiveRecord
+  module AttributeDecorators # :nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      class_attribute :attribute_type_decorations, instance_accessor: false # :internal:
+      self.attribute_type_decorations = TypeDecorator.new
+    end
+
+    module ClassMethods # :nodoc:
+      def decorate_attribute_type(column_name, decorator_name, &block)
+        matcher = ->(name, _) { name == column_name.to_s }
+        key = "_#{column_name}_#{decorator_name}"
+        decorate_matching_attribute_types(matcher, key, &block)
+      end
+
+      def decorate_matching_attribute_types(matcher, decorator_name, &block)
+        clear_caches_calculated_from_columns
+        decorator_name = decorator_name.to_s
+
+        # Create new hashes so we don't modify parent classes
+        self.attribute_type_decorations = attribute_type_decorations.merge(decorator_name => [matcher, block])
+      end
+
+      private
+
+      def add_user_provided_columns(*)
+        super.map do |column|
+          decorated_type = attribute_type_decorations.apply(column.name, column.cast_type)
+          column.with_type(decorated_type)
+        end
+      end
+    end
+
+    class TypeDecorator # :nodoc:
+      delegate :clear, to: :@decorations
+
+      def initialize(decorations = {})
+        @decorations = decorations
+      end
+
+      def merge(*args)
+        TypeDecorator.new(@decorations.merge(*args))
+      end
+
+      def apply(name, type)
+        decorations = decorators_for(name, type)
+        decorations.inject(type) do |new_type, block|
+          block.call(new_type)
+        end
+      end
+
+      private
+
+      def decorators_for(name, type)
+        matching(name, type).map(&:last)
+      end
+
+      def matching(name, type)
+        @decorations.values.select do |(matcher, _)|
+          matcher.call(name, type)
+        end
+      end
+    end
+  end
+end