gistfile1.rb

class Foo < ActiveRecord::Base
    has_many :foo_bars
    has_many :bars, :through => :foo_bars
end

class Bar < ActiveRecord::Base
end

class FooBar < ActiveRecord::Base
    belongs_to :foo
    belongs_to :bar
end

old_foo = Foo.find(id)
new_foo = Foo.create

# will attributes on the join models (FooBar) be copied or
# will they assume their default values?
new_foo.bars = old_foo.bars