The default proc for a model's field should be instance_exec'd on the model object (after all the mass-assignments are mass-assigned).

class Pet
include Mongoid::Document

field :ferocious,
:type => Boolean,
:default => false

field :name,
:type => String,
:default => proc

{ ferocious? ? "Jaws" : "Sparky" }

attr_accessible :ferocious, :name
end

describe Pet do
context "tame" do
subject

{ Pet.new }

it

{ should_not be_ferocious }

its(:name)

{ should == "Sparky" }

end
context "ferocious" do
subject

{ Pet.new(:ferocious => true) }

it

{ should be_ferocious }

its(:name)

{ should == "Jaws" }

end
end