mongoid-3.1.3, email and password fields in the following examples are required.

user = User.where(email: 'user1@example.com').first_or_create do |u|
u.password = '12345678'
end

Expected user.persisted? to be true
Got false

Raising validation error when using with a bang:

Mongoid::Errors::Validations:
Problem:
Validation of Referrer failed.
Summary:
The following errors were found: Password can't be blank
Resolution:
Try persisting the document with valid data or remove the validations.

While the following works as expected:

r = User.create do |u|
u.email = 'user1@example.com'
u.password = '12345678'
end

User.where(email: 'user1@example.com').first_or_create(password: '12345678')