Rails Rich Associations - undefined method error

I have three basic models that I am working with:

class User < ActiveRecord::Base
  has_many :assignments
end

class Assignment < ActiveRecord::Base
  belongs_to :group
  belongs_to :user  
end

class Group < ActiveRecord::Base
  has_many :assignments
end

Using this schema, I would assume the "Assignment" model is sort of the join table, which holds the information for which users belong to which groups. So, what I am trying to do is, using a User object, find out what groups they belong to.

In Rail console, I am doing the following:

me = User.find(1)

Which returns the user object, as it should. Then, I attempt to see which "groups" this user belongs to, which I thought it would go through the "Assignment" model. But, I'm obviously doing something wrong:

me.groups

Which returns:

NoMethodError: undefined method `groups' for #<User:0x007fd5d6320c68>

How would I go about finding out which "groups" the "me" object belongs to?

Thanks very much!

Solution

You have to declare the User - Groups relation in each model:

class User < ActiveRecord::Base
  has_many :assignments
  has_many :groups, through: :assignments
end

class Group < ActiveRecord::Base
  has_many :assignments
  has_many :users, through: :assignments
end

Also, I recommend you to set some validations on the Assignment model to make sure an Assignment always refers to a Group AND a User:

class Assignment < ActiveRecord::Base
  belongs_to :group
  belongs_to :user  
  validates :user_id, presence: true
  validates :group_id, presence: true
end