custom RSpec 3 matcher for Grape Entities

grape_entity_matcher.rb

# Custom RSpec 3 matcher to easily test validity of Grape API responses based on Grape::Entity serializers.
# It is assumed that all entities are defined as nested classes, which is the recommended approach, see
# the last example at https://github.com/intridea/grape#grape-entities
#
# HOW TO USE
# ----------
#
# 1. Require this file in your test environment.
#
# 2. To verify that a given JSON hash is the correct representation of a single model:
#       expect(json['user']).to represent_entity_of(user)
#
# 3. To verify that a given JSON array of hashes is the correct representation of an array of models:
#       expect(json['users']).to represent_array_of(users)
#

class GrapeEntityMatcher < Struct.new(:target)
  def represents?(actual)
    klass = target.class
    if !klass.const_defined?('Entity')
      raise ArgumentError, "#{klass} does not have nested Entity"
    end
    entity = klass.const_get('Entity').new(target)
    actual == entity.serializable_hash.stringify_keys
  end
end

RSpec::Matchers.define :represent_entity_of do |target|
  match do |actual|
    GrapeEntityMatcher.new(target).represents? actual
  end
end

RSpec::Matchers.define :represent_array_of do |targets|
  match do |actual_orig|
    return false if actual.size != targets.size

    actual = actual_orig.clone
    targets.all? do |item|
      match = actual.find { |t| GrapeEntityMatcher.new(item).represents? t }
      match ? (actual.delete(match) && true) : false
    end
  end
end