Question

In active_record/base.rb, module ActiveRecord you can see this code:

delegate :find, :first, :last, :all, :destroy, :destroy_all, :exists?, :delete, 
             :delete_all, :update, :update_all, :to => :scoped

Let’s take first method, so i assumed that first method delegates to scoped method and then scoped should return the first record from database. But scoped is just the anonymous scope, how the current construction is doing its job?

At the same time, how dynamic methods work, like find_by_name, find_all_by_name_and_colour?

Thanks

Answer 1

According to the documentation, delegate:

Provides a delegate class method to
easily expose contained objects’
methods as your own. Pass one or more
methods (specified as symbols or
strings) and the name of the target
object via the :to option (also a
symbol or string)

So this delegates the methods in the list to the scoped method, which is defined in ActiveRecord::NamedScoped::ClassMethods, and which returns an anonymous scope.

As to why ActiveRecord does this, it is so we can continue to use familiar methods such as find while behind the scenes AR is actually calling the fancy new Arel methods. For instance, when you do

Post.find(37)

What actually gets executed is:

Post.where(primary_key.eq(37))