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]: #1011 (comment)

active_model_serializers.gemspec

@@ -41,6 +41,8 @@ Gem::Specification.new do |spec|
     # 'minitest'
     # 'thread_safe'
 
+  spec.add_runtime_dependency 'json-schema'
+
   # Soft dependency for pagination
   spec.add_development_dependency 'kaminari', ' ~> 0.16.3'
   spec.add_development_dependency 'will_paginate', '~> 3.0', '>= 3.0.7'

lib/active_model/serializer/assertions.rb

@@ -0,0 +1,16 @@
+require 'json-schema'
+
+module ActiveModel
+  class Serializer
+    module Assertions
+      def assert_response_schema(schema_path = nil)
+        controller_path = response.request.filtered_parameters[:controller]
+        action = response.request.filtered_parameters[:action]
+        schema_directory = ActiveModel::Serializer.config.schema_path
+        schema_path ||= "#{controller_path}/#{action}.json"
+        schema_full_path = "#{schema_directory}/#{schema_path}"
+        JSON::Validator.validate!(schema_full_path, response.body, strict: true)
+      end
+    end
+  end
+end

lib/active_model/serializer/configuration.rb

@@ -10,6 +10,7 @@ module Configuration
         base.config.array_serializer = ActiveModel::Serializer::ArraySerializer
         base.config.adapter = :attributes
         base.config.jsonapi_resource_type = :plural
+        base.config.schema_path = 'test/support/schemas'
       end
     end
   end

lib/active_model_serializers.rb

@@ -44,13 +44,15 @@ def silence_warnings
 require 'active_model/serializer'
 require 'active_model/serializable_resource'
 require 'active_model/serializer/version'
+require 'active_model/serializer/assertions'
 
 require 'action_controller/serialization'
 ActiveSupport.on_load(:action_controller) do
   include ::ActionController::Serialization
   ActionDispatch::Reloader.to_prepare do
     ActiveModel::Serializer.serializers_cache.clear
   end
+  ActionController::TestCase.send(:include, ActiveModel::Serializer::Assertions)
 end
 
 require 'active_model/serializer/railtie'

test/assertions_test.rb

@@ -0,0 +1,46 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class AssertionsTest < ActionController::TestCase
+      class MyController < ActionController::Base
+        def index
+          render json: Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
+        end
+
+        def show
+          index
+        end
+      end
+
+      tests MyController
+
+      def test_that_assert_with_a_valid_schema
+        get :index
+        assert_response_schema
+      end
+
+      def test_that_raises_a_json_schema_with_a_invalid_schema
+        get :show
+        assert_raises JSON::Schema::ValidationError do
+          assert_response_schema
+        end
+      end
+
+      def test_that_assert_with_a_custom_schema
+        get :show
+        assert_response_schema('custom/show.json')
+      end
+
+      def test_that_assert_with_a_custom_schema_directory
+        original_schema_path = ActiveModel::Serializer.config.schema_path
+        ActiveModel::Serializer.config.schema_path = 'test/support/custom_schemas'
+
+        get :index
+        assert_response_schema
+
+        ActiveModel::Serializer.config.schema_path = original_schema_path
+      end
+    end
+  end
+end

test/support/custom_schemas/active_model/serializer/assertions_test/my/index.json

@@ -0,0 +1,6 @@
+{
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}

test/support/schemas/active_model/serializer/assertions_test/my/index.json

@@ -0,0 +1,6 @@
+{
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}

test/support/schemas/active_model/serializer/assertions_test/my/show.json

@@ -0,0 +1,5 @@
+{
+  "properties": {
+    "name" : { "type" : "string" }
+  }
+}

test/support/schemas/custom/show.json

@@ -0,0 +1,6 @@
+{
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}