Project defined attribute types should be valid

[CristianCurteanu]

It can be handy to add valid type to attributes that are defined within a project scope. In this pull request I have tried to make such functionality possible, and to make possible to add value types for classes that are defined. If the type/class is not defined, then UnsupportedTypeError exception still should be thrown

Also, there was made some refactoring to ModelAttribute::Casts module class methods, to be a bit more decoupled from .cast method

[msftclas]

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[dwaller]

Hi, thanks for the contribution. Can you explain a little more what you're trying to achieve with your change?

I'm a little concerned that what you're trying to do might be in opposition to what the gem's initial aims are - to support creating a model that fronts an HTTP web service. Particularly (from the readme):

• Handles integers, booleans, strings and times - a set of types that are very easy to persist to and parse from JSON.
• Supports efficient serialization of attributes to JSON.

I think you're trying to allow attributes that have arbitrary Ruby types. But how are these then serialized to JSON? JSON doesn't allow arbitrary types.

If you wanted to have an address attribute with Address type, for example, then I'd suggest setting the attribute to be JSON type, and provide your own accessors on the model that handle converting an Address instance to a JSON friendly hash. If you build the Address model using ModelAttribute then Address#attributes_for_json will easily give you a JSON friendly hash, and Address#set_attributes will allow you to turn that hash back into a populated model.

[CristianCurteanu]

Hello! Actually instead of Address type there could be set any defined class object, and the purpose of this change is to make it possible to use types that were defined within a scope. If the class is not defined then UnsupportedTypeError will be thrown.

[dwaller]

Whatever the class of the object you want to use, it will still not have a simple conversion to JSON primitives. This is where your change goes against the intent of this gem.