RFC: Json Api Errors (WIP)

- ActiveModelSerializers::JsonPointer
- ActiveModel::Serializer::Adapter::JsonApi::Error
- ActiveModel::Serializer::Adapter::JsonApi::Error.attributes
- Fix rubocop config

lib/active_model/serializable_resource.rb

@@ -16,6 +16,19 @@ module ActiveModel
       @resource = resource
       @adapter_opts, @serializer_opts =
         options.partition { |k, _| ADAPTER_OPTION_KEYS.include? k }.map { |h| Hash[h] }
+
+      # TECHDEBT: clean up single vs. collection of resources
+      if resource.respond_to?(:each)
+        if resource.any? { |elem| elem.respond_to?(:errors) && !elem.errors.empty? }
+          @serializer_opts[:serializer] = ActiveModel::Serializer::ErrorSerializer
+          @adapter_opts[:adapter] = :'json_api/error'
+        end
+      else
+        if resource.respond_to?(:errors) && !resource.errors.empty?
+          @serializer_opts[:serializer] = ActiveModel::Serializer::ErrorSerializer
+          @adapter_opts[:adapter] = :'json_api/error'
+        end
+      end
     end
 
     def serialization_scope=(scope)

lib/active_model/serializer.rb

@@ -1,6 +1,7 @@
 require 'thread_safe'
 require 'active_model/serializer/collection_serializer'
 require 'active_model/serializer/array_serializer'
+require 'active_model/serializer/error_serializer'
 require 'active_model/serializer/include_tree'
 require 'active_model/serializer/associations'
 require 'active_model/serializer/attributes'

lib/active_model/serializer/error_serializer.rb

@@ -0,0 +1,2 @@
+class ActiveModel::Serializer::ErrorSerializer < ActiveModel::Serializer
+end

lib/active_model/serializer/lint.rb

@@ -129,6 +129,20 @@ module ActiveModel::Serializer::Lint
       assert_instance_of resource_class.model_name, ActiveModel::Name
     end
 
+    def test_active_model_errors
+      assert_respond_to resource, :errors
+    end
+
+    def test_active_model_errors_human_attribute_name
+      assert_respond_to resource.class, :human_attribute_name
+      assert_equal(-2, resource.class.method(:human_attribute_name).arity)
+    end
+
+    def test_active_model_errors_lookup_ancestors
+      assert_respond_to resource.class, :lookup_ancestors
+      assert_equal 0, resource.class.method(:lookup_ancestors).arity
+    end
+
     private
 
     def resource

lib/active_model_serializers.rb

@@ -10,6 +10,7 @@ module ActiveModelSerializers
   autoload :Logging
   autoload :Test
   autoload :Adapter
+  autoload :JsonPointer
 
   class << self; attr_accessor :logger; end
   self.logger = ActiveSupport::TaggedLogging.new(ActiveSupport::Logger.new(STDOUT))

lib/active_model_serializers/adapter/json_api.rb

@@ -8,6 +8,7 @@ module ActiveModelSerializers
       require 'active_model/serializer/adapter/json_api/meta'
       autoload :Deserialization
       require 'active_model/serializer/adapter/json_api/api_objects'
+      autoload :Error
 
       # TODO: if we like this abstraction and other API objects to it,
       # then extract to its own file and require it.

lib/active_model_serializers/adapter/json_api/error.rb

@@ -0,0 +1,92 @@
+module ActiveModelSerializers
+  module Adapter
+    class JsonApi < Base
+      class Error < Base
+=begin
+## http://jsonapi.org/format/#document-top-level
+
+A document MUST contain at least one of the following top-level members:
+
+- data: the document's "primary data"
+- errors: an array of error objects
+- meta: a meta object that contains non-standard meta-information.
+
+The members data and errors MUST NOT coexist in the same document.
+
+## http://jsonapi.org/format/#error-objects
+
+Error objects provide additional information about problems encountered while performing an operation. Error objects MUST be returned as an array keyed by errors in the top level of a JSON API document.
+
+An error object MAY have the following members:
+
+- id: a unique identifier for this particular occurrence of the problem.
+- links: a links object containing the following members:
+- about: a link that leads to further details about this particular occurrence of the problem.
+- status: the HTTP status code applicable to this problem, expressed as a string value.
+- code: an application-specific error code, expressed as a string value.
+- title: a short, human-readable summary of the problem that SHOULD NOT change from occurrence to occurrence of the problem, except for purposes of localization.
+- detail: a human-readable explanation specific to this occurrence of the problem.
+- source: an object containing references to the source of the error, optionally including any of the following members:
+-  pointer: a JSON Pointer [RFC6901] to the associated entity in the request document [e.g. "/data" for a primary data object, or "/data/attributes/title" for a specific attribute].
+-  parameter: a string indicating which query parameter caused the error.
+- meta: a meta object containing non-standard meta-information about the error.
+
+=end
+        def self.attributes(attribute_name, attribute_errors)
+          attribute_errors.map do |attribute_error|
+            {
+              source: { pointer: ActiveModelSerializers::JsonPointer.new(:attribute, attribute_name) },
+              detail: attribute_error
+            }
+          end
+        end
+
+        def serializable_hash(*)
+          @result = []
+          # TECHDEBT: clean up single vs. collection of resources
+          if serializer.object.respond_to?(:each)
+            @result = collection_errors.flat_map do |collection_error|
+              collection_error.flat_map do |attribute_name, attribute_errors|
+                attribute_error_objects(attribute_name, attribute_errors)
+              end
+            end
+          else
+            @result = object_errors.flat_map do |attribute_name, attribute_errors|
+              attribute_error_objects(attribute_name, attribute_errors)
+            end
+          end
+          { root => @result }
+        end
+
+        def fragment_cache(cached_hash, non_cached_hash)
+          JsonApi::FragmentCache.new.fragment_cache(root, cached_hash, non_cached_hash)
+        end
+
+        def root
+          'errors'.freeze
+        end
+
+        private
+
+        # @return [Array<symbol, Array<String>] i.e. attribute_name, [attribute_errors]
+        def object_errors
+          cache_check(serializer) do
+            serializer.object.errors.messages
+          end
+        end
+
+        def collection_errors
+          cache_check(serializer) do
+            serializer.object.flat_map do |elem|
+              elem.errors.messages
+            end
+          end
+        end
+
+        def attribute_error_objects(attribute_name, attribute_errors)
+          Error.attributes(attribute_name, attribute_errors)
+        end
+      end
+    end
+  end
+end

lib/active_model_serializers/json_pointer.rb

@@ -0,0 +1,14 @@
+module ActiveModelSerializers
+  module JsonPointer
+    module_function
+
+    POINTERS = {
+      attribute:    '/data/attributes/%s'.freeze,
+      primary_data: '/data'.freeze
+    }.freeze
+
+    def new(pointer_type, value = nil)
+      format(POINTERS[pointer_type], value)
+    end
+  end
+end

lib/active_model_serializers/model.rb

@@ -6,10 +6,11 @@ module ActiveModelSerializers
     include ActiveModel::Model
     include ActiveModel::Serializers::JSON
 
-    attr_reader :attributes
+    attr_reader :attributes, :errors
 
     def initialize(attributes = {})
       @attributes = attributes
+      @errors = ActiveModel::Errors.new(self)
       super
     end
 
@@ -35,5 +36,14 @@ module ActiveModelSerializers
         attributes[key]
       end
     end
+
+    # The following methods are needed to be minimally implemented for ActiveModel::Errors
+    def self.human_attribute_name(attr, _options = {})
+      attr
+    end
+
+    def self.lookup_ancestors
+      [self]
+    end
   end
 end