Allows it to test accepts_nested_attributes_for with associations that are a `has_one`

spec_helper.rb

# See: https://gist.github.com/1353500
# Use: it { should accept_nested_attributes_for(:association_name).using_has_one(true).and_accept({valid_values => true}).but_reject({ :reject_if_nil => nil })}
RSpec::Matchers.define :accept_nested_attributes_for do |association|
  match do |model|
    @model = model
    @nested_att_present = model.respond_to?("#{association}_attributes=".to_sym)
    if @nested_att_present && @reject
      model.send("#{association}_attributes=".to_sym,[@reject])
      @reject_success = model.send("#{association}").empty?
    end
    # Let us use has_one associations
    model.send("#{association}").clear unless @using_has_one
    if @nested_att_present && @accept
      model.send("#{association}_attributes=".to_sym,[@accept])
      @accept_success = ! (model.send("#{association}").empty?)
    end
    @nested_att_present && ( @reject.nil? || @reject_success ) && ( @accept.nil? || @accept_success )
  end

  failure_message_for_should do
    messages = []
    messages << "accept nested attributes for #{association}" unless @nested_att_present
    messages << "reject values #{@reject.inspect} for association #{association}" unless @reject_success
    messages << "accept values #{@accept.inspect} for association #{association}" unless @accept_success
    "expected #{@model.class} to " + messages.join(", ")
  end

  description do
    desc = "accept nested attributes for #{expected}"
    if @reject
      desc << ", but reject if attributes are #{@reject.inspect}"
    end
  end

  chain :but_reject do |reject|
    @reject = reject
  end

  chain :and_accept do |accept|
    @accept = accept
  end

  chain :using_has_one do |using_has_one|
    @using_has_one = using_has_one
  end
end