如何在Rails中关注的内部使用Attribute API？

Question

I have a simple generic model rails that looks like this:

class Thing < ApplicationRecord
  attribute :foo, :integer
  include AConcern
end

And it include a basic concern that looks like this…

module AConcern
  extend ActiveSupport::Concern
end

The model also has an attribute called :foo using the attribute api from below:

https://api.rubyonrails.org/classes/ActiveRecord/Attributes/ClassMethods.html

The attribute relates to the the concern, so every time I want to use the concern, in each model I have to define the attribute and then include the concern.

If I put the attribute declration inside the concern like this:

module AConcern
  extend ActiveSupport::Concern
  attribute :foo, :integer
end

I get the following error:

undefined method `attribute' for AConcern:Module

How do I use the attribute definition in the concern so I don't have to declare it in every model before including the concern? Thanks

Answer 1

You can use ActiveSupport::Concern included hook to handle this eg

module AConcern
  extend ActiveSupport::Concern
  included do 
    attribute :foo, :integer
  end
end 

Then

class Thing < ApplicationRecord
  include AConcern
end

The problem you are experiencing right now is that attribute is being called in the context of your Module but that module does not have access to that method (hence NoMethodError ).

The included hook is run when you call include and the hook is run in the context of the including Object ( Thing in this case). Thing does have the attribute method and thus everything works as expected.

included block from ActiveSupport::Concern is essentially the same as (in pure ruby)

module AConcern
  def self.included(base) 
    base.class_eval { attribute :foo, :integer } 
  end 
end