Dont quite follow this, does the build allow you to add fields?

Are you saying this:

FactoryGirl.define do
  factory :user do
    name        { Faker::Name.name }
    email       { Faker::Internet.email }
    nickname    ''
    challenge
    commitment  1000
  end
end

to

FactoryGirl.define do
  factory :user do
    name        { Faker::Name.name }
    email       { Faker::Internet.email }
    challenge
    commitment  1000
  end
end

Then use your examples above?