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

test/active_model_serializers/test/schema_test.rb

@@ -0,0 +1,128 @@
+require 'test_helper'
+
+module ActiveModelSerializers
+  module Test
+    class SchemaTest < ActionController::TestCase
+      include ActiveModelSerializers::Test::Schema
+
+      class MyController < ActionController::Base
+        def index
+          render json: profile
+        end
+
+        def show
+          index
+        end
+
+        def name_as_a_integer
+          profile.name = 1
+          index
+        end
+
+        def render_using_json_api
+          render json: profile, adapter: :json_api
+        end
+
+        def invalid_json_body
+          render json: ''
+        end
+
+        private
+
+        def profile
+          @profile ||= Profile.new(name: 'Name 1', description: 'Description 1', comments: 'Comments 1')
+        end
+      end
+
+      tests MyController
+
+      def test_that_assert_with_a_valid_schema
+        get :index
+        assert_response_schema
+      end
+
+      def test_that_raises_a_minitest_error_with_a_invalid_schema
+        message = "#/name: failed schema #/properties/name: For 'properties/name', \"Name 1\" is not an integer. and #/description: failed schema #/properties/description: For 'properties/description', \"Description 1\" is not a boolean."
+
+        get :show
+
+        error = assert_raises Minitest::Assertion do
+          assert_response_schema
+        end
+        assert_equal(message, error.message)
+      end
+
+      def test_that_raises_error_with_a_custom_message_with_a_invalid_schema
+        message = 'oh boy the show is broken'
+
+        get :show
+
+        error = assert_raises Minitest::Assertion do
+          assert_response_schema(nil, message)
+        end
+        assert_equal(message, error.message)
+      end
+
+      def test_that_assert_with_a_custom_schema
+        get :show
+        assert_response_schema('custom/show.json')
+      end
+
+      def test_that_assert_with_a_hyper_schema
+        get :show
+        assert_response_schema('hyper_schema.json')
+      end
+
+      def test_simple_json_pointers
+        get :show
+        assert_response_schema('simple_json_pointers.json')
+      end
+
+      def test_simple_json_pointers_that_doesnt_match
+        get :name_as_a_integer
+
+        assert_raises Minitest::Assertion do
+          assert_response_schema('simple_json_pointers.json')
+        end
+      end
+
+      def test_json_api_schema
+        get :render_using_json_api
+        assert_response_schema('render_using_json_api.json')
+      end
+
+      def test_that_assert_with_a_custom_schema_directory
+        original_schema_path = ActiveModelSerializers.config.schema_path
+        ActiveModelSerializers.config.schema_path = 'test/support/custom_schemas'
+
+        get :index
+        assert_response_schema
+
+        ActiveModelSerializers.config.schema_path = original_schema_path
+      end
+
+      def test_with_a_non_existent_file
+        message = %r{.* - No Schema file at test/support/schemas/non-existent.json}
+
+        get :show
+
+        error = assert_raises ActiveModelSerializers::Test::Schema::MissingSchema do
+          assert_response_schema('non-existent.json')
+        end
+        assert_match(message, error.message)
+      end
+
+      def test_that_raises_with_a_invalid_json_body
+        message = 'A JSON text must at least contain two octets!'
+
+        get :invalid_json_body
+
+        error = assert_raises ActiveModelSerializers::Test::Schema::InvalidSchemaError do
+          assert_response_schema('custom/show.json')
+        end
+
+        assert_equal(message, error.message)
+      end
+    end
+  end
+end

test/support/custom_schemas/active_model_serializers/test/schema_test/my/index.json

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

test/support/schemas/active_model_serializers/test/schema_test/my/index.json

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

test/support/schemas/active_model_serializers/test/schema_test/my/show.json

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

test/support/schemas/custom/show.json

@@ -0,0 +1,7 @@
+{
+  "id": "file://custom/show.json#",
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}

test/support/schemas/hyper_schema.json

@@ -0,0 +1,93 @@
+{
+  "$schema": "http://json-schema.org/draft-04/hyper-schema",
+  "title": "Profile",
+  "description": "Profile schema",
+  "stability": "prototype",
+  "strictProperties": true,
+  "type": [
+    "object"
+  ],
+  "definitions": {
+    "name": {
+      "description": "unique name of profile",
+      "readOnly": true,
+      "type": [
+        "string"
+      ]
+    },
+    "description": {
+      "description": "description of profile",
+      "readOnly": true,
+      "type": [
+        "string"
+      ]
+    },
+    "identity": {
+      "anyOf": [
+        {
+          "$ref": "/schemata/profile#/definitions/name"
+        }
+      ]
+    }
+  },
+  "links": [
+    {
+      "description": "Create a new profile.",
+      "href": "/profiles",
+      "method": "POST",
+      "rel": "create",
+      "schema": {
+        "properties": {
+        },
+        "type": [
+          "object"
+        ]
+      },
+      "title": "Create"
+    },
+    {
+      "description": "Delete an existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "DELETE",
+      "rel": "destroy",
+      "title": "Delete"
+    },
+    {
+      "description": "Info for existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "GET",
+      "rel": "self",
+      "title": "Info"
+    },
+    {
+      "description": "List existing profiles.",
+      "href": "/profiles",
+      "method": "GET",
+      "rel": "instances",
+      "title": "List"
+    },
+    {
+      "description": "Update an existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "PATCH",
+      "rel": "update",
+      "schema": {
+        "properties": {
+        },
+        "type": [
+          "object"
+        ]
+      },
+      "title": "Update"
+    }
+  ],
+  "properties": {
+    "name": {
+      "$ref": "/schemata/profile#/definitions/name"
+    },
+    "description": {
+      "$ref": "/schemata/profile#/definitions/description"
+    }
+  },
+  "id": "/schemata/profile"
+}

test/support/schemas/render_using_json_api.json

@@ -0,0 +1,43 @@
+{
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "id": "",
+  "type": "object",
+  "properties": {
+    "data": {
+      "id": "/data",
+      "type": "object",
+      "properties": {
+        "id": {
+          "id": "/data/id",
+          "type": "string"
+        },
+        "type": {
+          "id": "/data/type",
+          "type": "string"
+        },
+        "attributes": {
+          "id": "/data/attributes",
+          "type": "object",
+          "properties": {
+            "name": {
+              "id": "/data/attributes/name",
+              "type": "string"
+            },
+            "description": {
+              "id": "/data/attributes/description",
+              "type": "string"
+            }
+          }
+        }
+      },
+      "required": [
+        "id",
+        "type",
+        "attributes"
+      ]
+    }
+  },
+  "required": [
+    "data"
+  ]
+}

test/support/schemas/simple_json_pointers.json

@@ -0,0 +1,10 @@
+{
+  "properties": {
+    "name": {
+      "$ref": "file://custom/show.json#/properties/name"
+    },
+    "description": {
+      "$ref": "file://custom/show.json#/properties/description"
+    }
+  }
+}