https://docs.mongodb.com/mongoid/master/tutorials/mongoid-relations/#dependent-behavior specifies that the default dependent behavior is to nullify. However, in this case:

class C
  include Mongoid::Document

  belongs_to :b
end

 

class B
  include Mongoid::Document

  has_many :cs, class_name: "C", inverse_of: :b
end

b=B.create!
c=C.create!(b:b)
b.destroy
c.reload
# => #<C _id: 5c5f7b9e026d7c4248d44e27, b_id: BSON::ObjectId('5c5f7b9e026d7c4248d44e26')>

... c still retained a b_id referencing the now nonexistent b.

If nullify is manually specified the behavior is correct:

class C
  include Mongoid::Document

  belongs_to :b
end

 

class B
  include Mongoid::Document

  has_many :cs, class_name: "C", inverse_of: :b, dependent: :nullify
end

b=B.create!
c=C.create!(b:b)
b.destroy
c.reload
# => #<C _id: 5c5f7bde026d7c43881dc718, b_id: nil>

If ActiveRecord's default behavior is nullify as well, Mongoid's behavior should be changed to nullify to match both our existing documentation and AR's behavior.

From ActiveRecord's documentation: https://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#module-ActiveRecord::Associations::ClassMethods-label-Dependent+associations

"When no option is given, the behavior is to do nothing with the associated records when destroying a record."

This seems to be consistent with how we handle the situation... Perhaps we should update the documentation to reflect this?