Rails: Validating association after save?

Question

I have a User model which has many roles. Roles contains a user_id field, which I want to validate_presence_of

The is

Answer 1

For anyone Googling for a solution to this problem for a has_many :through association, as of 5/December/2013 the :inverse_of option can't be used in conjunction with :through (source). Instead, you can use the approach suggested by @waldyr.ar. For example, if our models are set up as follows:

class User < ActiveRecord::Base
  has_many :roles
  has_many :tasks, through: roles
end

class Role < ActiveRecord::Base  
  belongs_to :user  
  belongs_to :task  
end

class Task < ActiveRecord::Base
  has_many :roles
  has_many :users, through: roles
end

We can modify our Role class as follows to validate the presence of both task and user before saving

class Role < ActiveRecord::Base  
  belongs_to :user  
  belongs_to :task  
  before_save { validates_presence_of :user, :task }
end

Now if we create a new User and add a couple tasks like so:

>> u = User.new  
>> 2.times { u.tasks << Task.new }

Running u.save will save the User and the Task, as well as transparently build and save a new Role whose foreign keys user_id and task_id are set appropriately. Validations will run for all models, and we can go on our merry way!