Document defining custom types

README.md

@@ -89,6 +89,7 @@ product.save
   * [Unknown attributes](./docs/unknown_attributes.md)
 3. [Array of stored models](./docs/array_of_stored_models.md)
 4. [Alternatives](./docs/alternatives.md)
+5. [Defining custom types](./docs/defining_custom_types.md)
 
 ## License
 

docs/defining_custom_types.md

@@ -0,0 +1,46 @@
+Firstly, you should define your custom type (for instance, they can be kept inside the `app/types` directory):
+
+```ruby
+class Iso8601Type < ActiveRecord::Type::Value
+  def type
+    :datetime_iso8601
+  end
+
+  def cast(value)
+    return value if value.is_a?(Time)
+    return value.to_time if value.is_a?(Date)
+    return nil if value.blank?
+
+    Time.iso8601(value)
+  rescue ArgumentError, TypeError
+    nil
+  end
+end
+
+```
+
+Secondly, register the type (initializer is a good place to do that):
+
+```ruby
+ActiveModel::Type.register(:datetime_iso8601, Iso8601Type)
+```
+
+Finally, use the type in your model:
+
+```ruby
+class Configuration
+  include StoreModel::Model
+
+  attribute :archived_at, :datetime_iso8601
+end
+```
+
+Alternatively, you can skip registering the type globally in a following way:
+
+```ruby
+class Configuration
+  include StoreModel::Model
+
+  attribute :archived_at, Iso8601Type.new
+end
+```