Question

Mongoid ships with .push on a habtm, which sets a habtm relationship in both directions. Although delete will #delete an associated record, there’s no documented way to delete only a relationship that I have seen. Is there a better way of doing this?

Is there a better way of ensuring uniqueness?

has_and_belongs_to_many :following, {class_name: 'User', inverse_of: :followers, inverse_class_name: 'User'}
  has_and_belongs_to_many :followers, {class_name: 'User', inverse_of: :following, inverse_class_name: 'User'}

  def follow!(user)
    self.following.push(user) # this pushes the inverse as well
    self.following_ids.uniq!
    self.save!
    user.follower_ids.uniq!
    user.save!
  end

  def unfollow!(user)
    self.following.delete(user.id)
    self.save!
    user.followers.delete(self.id)
    user.save!
  end

Answer 1

Following code worked fine for me (mongoid 2.3.x):

class User
  include Mongoid::Document

  field :name, type: String

  has_and_belongs_to_many :following, class_name: 'User', inverse_of: :followers, autosave: true
  has_and_belongs_to_many :followers, class_name: 'User', inverse_of: :following

  def follow!(user)
    if self.id != user.id && !self.following.include?(user)
      self.following << user
    end
  end

  def unfollow!(user)
    self.following.delete(user)
  end
end

No inverse_class_name, no save calls, no special handling, but with exclusion of self-following.

The reason is, that mongoid automatically uses dependent: nullify if not added to the relation statement. And with autosave: true the update of relationships get saved (and is only needed for following, because we do not alter followers directly). Without autosave option you need to add a save call in the methods, because mongoid doesn’t automatically save relationship updates (since 2.0.0.x).

I put the if-clause as block, so you can alter it with exception handling (else raise FooException).

The .delete(user) is okay, also mentioned in the mongoid docs: http://mongoid.org/docs/relations/referenced/n-n.html (scroll down to “DEPENDENT BEHAVIOUR”).