fixtures.rb

## Fixjour

define_builder(Person) do |klass, overrides|
  overrides.process(:child) do |is_child|
    overrides[:dob] = is_child ? 2.years.ago : 42.years.ago
  end

  klass.new(:name => "Pat")
end

create_person

create_person(:child => true)

## FixtureReplacement

attributes_for :person do |p|
  p.name = "Pat"
end

attributes_for :child, :from => :person, :class => Person do |c|
  c.child = true
end

attributes_for :adult, :from => :person, :class => Person do |a|
  a.child = false
end

create_person() #=> <Person#...>
create_child() #=> <Person# child = true>
create_adult() #=> <Person# child = false)

create_person(:child => true)