Let's say I have an application with a has_and_belongs_to_many relation between a Client model and a Company model.

In the Client model there is a validation to make sure the object is associated with at least one company (companies cannot be blank).

The thing is, when I try to update a client with an empty list of companies, despite raising a validation error message, the invalid model is updated in the database.

Is this an expected behavior?

One can reproduce it by running the following code:

class Company
  include Mongoid::Document

  field :name, type: String
end

class Client
  include Mongoid::Document

  field :name, type: String

  has_and_belongs_to_many :companies, class_name: 'Company'

  validates :companies, presence: true
end

c = Client.first
c.update!(companies: []) # raises error but updates document