class Foo
  include Mongoid::Document

  has_many :dependencies

  def add_dependency
    self.dependencies << Dependency.new
  end

end

class Dependency
  include Mongoid::Document

  belongs_to :foo

  field :name

  validates_presence_of :name
end

Foo.create!

f = Foo.first
f.add_dependency # won't save foo because of the dependency validation
f2 = Foo.find(f.id)
f2.dependencies
#=> [#<Dependency _id: 513a534650b3683fcf000006, foo_id: "513a52f650b3683fcf000005", name: nil>]
Dependency.count
#=> 0

f3 = Foo.last
f3.dependencies
#=> []

f.save
#=> false