custom_rspec_matcher.rb

# The actual matcher class
class VirtusAttributeMatcher
  # initialize is called with the expected outcome
  # => expect(described_class).to have_attribute(:token)
  def initialize(attribute_name)
    @attribute_name = attribute_name
    @options = {}
  end

  # When RSpec will call matches? on the expectation class, this needs to
  # return true if everything is good, or false if there are errors
  def matches?(subject)
    @subject = subject
    attribute_exists? && type_correct?
  end

  # Extra methods must return self in order for RSpec to call matches? on the final object
  def of_type(type)
    @options[:type] = type
    self
  end

  # The message that is given to failures (expect(described_class).to)
  # Make this helpful so people know why it failed.
  def failure_message
    "expected #{@attribute_name} to be defined"
  end

  # The message that is given to negative failures (expect(described_class).to_not)
  # Make this helpful so people know why it failed.
  def negative_failure_message
    "expected #{@attribute_name} not to be defined"
  end

  private

  def attribute
    @subject.attribute_set[@attribute_name]
  end

  def attribute_exists?
    attribute != nil
  end

  def type_correct?
    if @options[:type]
      attribute.options[:primitive] == @options[:type]
    else
      true
    end
  end
end

# The module with methods that will be mixed into RSpec, this will provide the interface
# from RSpec to your matcher class
module VirtusRspecMatchers
  def have_attribute(attribute_name)
    VirtusAttributeMatcher.new(attribute_name)
  end
end

# Mix in the matchers
RSpec.configure do |config|
  config.include VirtusRspecMatchers
end