Rails: Overriding ActiveRecord association method

Question

Is there a way to override one of the methods provided by an ActiveRecord association?

Say for example I have the following typical polymorphic has_many :through ass

Answer 1

If you want to access the model itself in Rails 3.2 you should use proxy_association.owner

Example:

class Author < ActiveRecord::Base
  has_many :books do
    def << (book)
      proxy_association.owner.add_book(book)
    end
  end

  def add_book (book)
    # do your thing here.
  end
end

See documentation

Answer 2

The method I use is to extend the association. You can see the way I handle 'quantity' attributes here: https://gist.github.com/1399762

It basically allows you to just do

has_many : tags, :through => : taggings, extend => QuantityAssociation

Without knowing exactly what your hoping to achieve by overriding the methods its difficult to know if you could do the same.

Answer 3

This may not be helpful in your case but could be useful for others looking into this.

Association Callbacks: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

Example from the docs:

class Project
  has_and_belongs_to_many :developers, :after_add => :evaluate_velocity

  def evaluate_velocity(developer)
    ...
  end
end

Also see Association Extensions:

class Account < ActiveRecord::Base
  has_many :people do
    def find_or_create_by_name(name)
      first_name, last_name = name.split(" ", 2)
      find_or_create_by_first_name_and_last_name(first_name, last_name)
    end
  end
end

person = Account.first.people.find_or_create_by_name("David Heinemeier Hansson")
person.first_name # => "David"
person.last_name  # => "Heinemeier Hansson"

Answer 4

I think you wanted def tags.<<(*new_tags) for the signature, which should work, or the following which is equivalent and a bit cleaner if you need to override multiple methods.

class << tags
  def <<(*new_tags)
    # rawr!
  end
end

Answer 5

Rails guides documents about overriding the added methods directly.

OP's issue with overriding << probably is the only exception to this, for which follow the top answer. But it wouldn't work for has_one's = assignment method or getter methods.

Answer 6

You would have to define the tags method to return an object which has a << method.

You could do it like this, but I really wouldn't recommend it. You'd be much better off just adding a method to your model that does what you want than trying to replace something ActiveRecord uses.

This essentially runs the default tags method adds a << method to the resulting object and returns that object. This may be a bit resource intensive because it creates a new method every time you run it

def tags_with_append
  collection = tags_without_append
  def collection.<< (*arguments)
    ...
  end
  collection
end
# defines the method 'tags' by aliasing 'tags_with_append'
alias_method_chain :tags, :append