Given the following model (with a unique index defined):

class Animal
include Mongoid::Document
field :name

index :name, :unique => true
create_indexes
end

Creating an initial record succeeds, and the record can be refound given its id:

first = Animal.create!
Animal.find(first.id)

Creating a second record appears to succeed. However, calling find with the record's id raises a Mongoid::Errors::DocumentNotFound exception; it was never persisted to mongodb.

second = Animal.create!
Animal.find(second.id)

Going into mongodb itself and inserting the document makes it clear that the second create! should fail - the unique constraint has been violated as both names are null ({{E11000 duplicate key error index: development.animals.$name_1 dup key: { : null }}}).