Create assert_response_schema test helper

It is a common pattern to use JSON Schema to validate a API response[1], [2]
and [3].

This patch creates the `assert_response_schema` test helper that helps people do
this kind of validation easily on the controller tests.

[1]: https://robots.thoughtbot.com/validating-json-schemas-with-an-rspec-matcher
[2]: https://github.com/sharethrough/json-schema-rspec
[3]: https://github.com/rails-api/active_model_serializers/issues/1011#issuecomment-127608121

lib/active_model/serializer/configuration.rb

@@ -21,6 +21,7 @@ module ActiveModel
 
         config.adapter = :attributes
         config.jsonapi_resource_type = :plural
+        config.schema_path = 'test/support/schemas'
       end
     end
   end

lib/active_model/serializer/railtie.rb

@@ -21,6 +21,7 @@ module ActiveModel
     end
 
     if Rails.env.test?
+      ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Schema)
       ActionController::TestCase.send(:include, ActiveModelSerializers::Test::Serializer)
     end
   end

lib/active_model_serializers/test.rb

@@ -2,5 +2,6 @@ module ActiveModelSerializers
   module Test
     extend ActiveSupport::Autoload
     autoload :Serializer
+    autoload :Schema
   end
 end

lib/active_model_serializers/test/schema.rb

@@ -0,0 +1,103 @@
+module ActiveModelSerializers
+  module Test
+    module Schema
+      # A Minitest Assertion that test the response is valid against a schema.
+      # @params schema_path [String] a custom schema path
+      # @params message [String] a custom error message
+      # @return [Boolean] true when the response is valid
+      # @return [Minitest::Assertion] when the response is invalid
+      # @example
+      #   get :index
+      #   assert_response_schema
+      def assert_response_schema(schema_path = nil, message = nil)
+        matcher = AssertResponseSchema.new(schema_path, response, message)
+        assert(matcher.call, matcher.message)
+      end
+
+      MissingSchema = Class.new(Errno::ENOENT)
+      InvalidSchemaError = Class.new(StandardError)
+
+      class AssertResponseSchema
+        attr_reader :schema_path, :response, :message
+
+        def initialize(schema_path, response, message)
+          require_json_schema!
+          @response = response
+          @schema_path = schema_path || schema_path_default
+          @message = message
+          @document_store = JsonSchema::DocumentStore.new
+          add_schema_to_document_store
+        end
+
+        def call
+          json_schema.expand_references!(store: document_store)
+          status, errors = json_schema.validate(response_body)
+          @message ||= errors.map(&:to_s).to_sentence
+          status
+        end
+
+        protected
+
+        attr_reader :document_store
+
+        def controller_path
+          response.request.filtered_parameters[:controller]
+        end
+
+        def action
+          response.request.filtered_parameters[:action]
+        end
+
+        def schema_directory
+          ActiveModelSerializers.config.schema_path
+        end
+
+        def schema_full_path
+          "#{schema_directory}/#{schema_path}"
+        end
+
+        def schema_path_default
+          "#{controller_path}/#{action}.json"
+        end
+
+        def schema_data
+          load_json_file(schema_full_path)
+        end
+
+        def response_body
+          load_json(response.body)
+        end
+
+        def json_schema
+          @json_schema ||= JsonSchema.parse!(schema_data)
+        end
+
+        def add_schema_to_document_store
+          Dir.glob("#{schema_directory}/**/*.json").each do |path|
+            schema_data = load_json_file(path)
+            extra_schema = JsonSchema.parse!(schema_data)
+            document_store.add_schema(extra_schema)
+          end
+        end
+
+        def load_json(json)
+          JSON.parse(json)
+        rescue JSON::ParserError => ex
+          raise InvalidSchemaError, ex.message
+        end
+
+        def load_json_file(path)
+          load_json(File.read(path))
+        rescue Errno::ENOENT
+          raise MissingSchema, "No Schema file at #{schema_full_path}"
+        end
+
+        def require_json_schema!
+          require 'json_schema'
+        rescue LoadError
+          raise LoadError, "You don't have json_schema installed in your application. Please add it to your Gemfile and run bundle install"
+        end
+      end
+    end
+  end
+end