mongoid key issue

gistfile1.rb

class Car
  include Mongoid::Document
  
  field :name
  field :colour
  
  key :name
end

c1 = Car.new :name => 'bob', :colour => 'red'
c1.save # returns true

c2 = Car.new :name => 'bob', :colour => 'blue'
c2.save # returns true

Car.all.count # returns 1
Car.first.name # returns 'bob'
Car.first.colour # returns 'red'