Rails - Nov 2011 - how to simulate typecasting of ActiveRecord in non-AR classes?

Hi,

I have non-ActiveRecord Ruby classes that I use for numerical (and
other) processing as part of an API service.

I seem to recall that AR will cast attributes, say, from a params hash,
into the data type expected for the attribute. That is useful, and I''m
missing that feature in my non-AR classes. Has anyone considered this
challenge in their own work, and, if so, what mechanism (existing
modules or code snippets) would they propose as a way to introduce
casting?

Thanks,

Grar

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On 04 Nov 2011, at 15:01, Grary Stimon wrote:
> I have non-ActiveRecord Ruby classes that I use for numerical (and
> other) processing as part of an API service.
>
> I seem to recall that AR will cast attributes, say, from a params  
> hash,
> into the data type expected for the attribute. That is useful, and
I''m
> missing that feature in my non-AR classes. Has anyone considered this
> challenge in their own work, and, if so, what mechanism (existing
> modules or code snippets) would they propose as a way to introduce
> casting?
Something along the lines of https://gist.github.com/1339540 (put it  
in the lib/ directory and include it in load path if needed) should  
work.

Then you just create a custom class, e.g. an ActiveModel class:

class CustomModel
   include ActiveModel::Validations
   include ActiveModel::Conversion
   include ActiveModel::Serializers::JSON
   include ActiveModel::Serializers::Xml
   extend ActiveModel::Naming

   include TypeCaster

   field :id, Integer
   field :name, String
   field :description, String
   field :some_integer, Integer

   validates_presence_of :id, :name
   validate_numericality_of :some_integer

   def persisted?
     false
   end
end

Just doing this off the top of my head, so might be some typos in  
there, but you get the point.

I''d use this method so your custom classes at least have some notion  
of what to expect, that''s what ActiveRecord also does (using the  
database column types).

You can then just pass in a json hash to the constructor:

CustomModel.new(params)

where params is a json hash like {name: "John Doe", id: 123,  
description: "Some anonymous man", some_integer: 5}

Even if for some reason that some_integer would be passed in as a  
string, your custom model would still typecast it to an integer, since  
your model enforces it to.


Best regards

Peter De Berdt

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On 04 Nov 2011, at 16:24, Peter De Berdt wrote:
> You can then just pass in a json hash to the constructor:
>
> CustomModel.new(params)
>
> where params is a json hash like {name: "John Doe", id: 123,  
> description: "Some anonymous man", some_integer: 5}
I stand corrected, where params is a normal Ruby hash (my weekend has  
started, my mind is slowing down ;-))


Best regards

Peter De Berdt

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Peter,

Wow, that''s a lot!

One of my reasons for posting was I saw 
ActiveModel::Serializers#from_json and I thought, ''What good is this,
if
there is not some other facility to handle non-String attributes?'' It 
seems, though, that the functionality you have so generously sketched is 
just not available via some overriding of AM modules.

Thanks,

Grar

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