assign_to_matcher.rb

RSpec::Matchers.define :assign_to do |variable|
  chain :with do |value|
    @value = value
  end

  match do |response|
    if @value
      assigns[variable] == @value
    else
      assigns.has_key?(variable)
    end
  end

  failure_message_for_should do |model|
    if @value
      "Subject:  @#{variable}
Expected: #{@value.inspect}
Actual:   #{assigns[variable].inspect}"
    else
      "Subject:  @#{variable}
Expected: anything
Actual:   not set"
    end
  end
end