Question

I have a standard polymorphic relation set up like:

#Foo
has_many: bar, as: foobar

#Qux
has_many: bar, as: foobar

#Bar
belongs_to :foobar, polymorphic: true

I want to be able to enforce a model that has_many: bar, as: foobar to have certain attributes for example a name, so I can do something like:

bar.foobar.name

and know that it doesn’t matter which model foobar is at that moment. (Since all foobars have a name attribute)

Is there a way to do this or do I have to test for it with responds_to? everytime I want to use it.

Answer 1

You can provide defaults by creating a base class that Foo and Qux extend. Instead of

class Foo < ActiveRecord::Base
  has_many: bar, as: foobar
end

class Qux < ActiveRecord::Base
  has_many: bar, as: foobar
end

you could do something like

class Foobar < ActiveRecord::Base
  has_many: bar, as: foobar
end

class Foo < Foobar; end

class Qux < Foobar; end

You could then create a default accessor method for the attributes you want to be consistent for those classes extending Foobar.

class Foobar < ActiveRecord::Base
  has_many: bar, as: foobar

  def name
    "some default name"
  end
end

class Foo < Foobar
  def name
    "some overloaded name for Foo"
  end
end

class Qux < Foobar; end

You can overload name as shown above in Foo, or omit it as shown in Qux. Regardless, a name attribute will be available, even if it’s only the default from Foobar.

foo = Foo.new
puts foo.name  # "some overloaded name for Foo"

qux = Qux.new
puts qux.name # "some default name"

If you’re looking to create more of a contract through an interface or abstract class as is common in other languages, Ruby doesn’t support either. You can however use a mixin that you’d include in Foo and Qux to throw an Exception if name is not defined. Read an example of this sort of implementation here.

Note on the chance these models are supposed to have an associated name column in the table, you will likely want to incorporate read_attribute(:name) somewhere in your overloaded name method. See the docs on read_attribute