Rails 4 HABTM custom validation on associations

Question

I've got a simple scenario, but I can't seem to find any proposed solutions that apply to Rails 4. I want to simply add a custom validator that checks the amount of stored associations between my HABTM association. Easier said and done, to my surprise? I've searched for a solution but only end up with answers for older versions of Rails it seems. I've got the following: ``` class User < ActiveRecord::Base has_and_belongs_to_many :roles after_save :check_maximum_number_of_roles . . . private def check_maximum_number_of_roles if self.roles.length > 3 errors.add(:roles, 'Users can only have three roles assigned.') return false end end end class Role < ActiveRecord::Base has_and_belongs_to_many :users end ``` The reason I use `after_save` is because as far as I understand the stored association is first available after it has been added. I've also tried to write an custom validator (e.g. `validate: :can_only_have_one_role`), but that does not work either. I add the association in the foll…

Accepted Answer

`user.roles << role` performs no validation on `user`. The user is largely uninvolved. All this does is insert a new record into your joining table.

If you want to enforce that a user has only one role, you have two options, both involve throwing away `has_and_belongs_to_many`, which you really shouldn't use anymore. Rails provides `has_many :through`, and that has been the preferred way of doing many-to-many relationship for some time.

So, the first (and I think best) way would be to use `has_many`/`belongs_to`. That is how you model one-to-many relationships in Rails. It should be this simple:

```
class Role
  has_many :users
end

class User
  belongs_to :role
end
```

The second way, which is over complex for enforcing a single associated record, is to create your joining model, call it `UserRole`, use a `has_many :through`, and perform the validation inside `UserRole`.

```
class User
  has_many :user_roles
  has_many :roles, through: :user_roles
end

class UserRole
  belongs_to :user
  belongs_to :role

# Validate that only one role exists for each user
  validates :user_id, uniqueness: { scope: :role_id }

# OR, to validate at most X roles are assigned to a user
  validate :at_most_3_roles, on: :create

def at_most_3_roles
    duplicates = UserRole.where(user_id: user_id, role_id: role_id).where('id != ?', id)
    if duplicates.count > 3
      errors.add(:base, 'A user may have at most three roles')
    end
  end
end

class Role
  has_many :user_roles
  has_many :users, through: :user_roles
end
```

Rails 4 HABTM custom validation on associations

Solution

Problem