How to use Squeel to make innested query?

I'd like to build a query builder using squeel, here is how I did:

Parent class:

module ModelFilters
  class Base
    def initialize(user)
      @user = user
    end

    def build(method_name)
      head.where { my { send(method_name) } }
    end

    protected

    def head
    end
  end
end

Child class:

module ModelFilters
  class Collection < Base
    def __showable__
      not_private | author
    end

    protected

    def head
      ::Collection.joins(:collaborators)
    end

    private

    def not_private
      is_private == false
    end

    def author
      user_id == @user.id
    end
  end
end

And finally my call:

a = ModelFilters::Collection.new(user)
a.build(:__showable__)

Actually my problem is that I don't know how Squeel work for innested query, my actual error is undefined local variable or method 'is_private' (obviously).

Is there a way to build something like this using Squeel (or other ActiveRecord query builder)?

Thank you all!

What if you do:

module ModelFilters
  class Base
    def initialize(user)
      @user = user
    end

    def build(method_name)
      head.where { |q| send(method_name, q) }
    end

    protected

    def head
    end
  end
end

module ModelFilters
  class Collection < Base
    def __showable__(q)
      not_private(q) | author(q)
    end

    protected

    def head
      ::Collection.joins(:collaborators)
    end

    private

    def not_private(q)
      q.is_private == false
    end

    def author(q)
      q.user_id == @user.id
    end
  end
end