custom entity IDs

lib/grape/jsonapi/entity/resource_identifier.rb

@@ -8,11 +8,28 @@ class << self
           attr_reader :type_plural
         end
 
+        def self.inherited(subclass)
+          class << subclass
+            attr_accessor :custom_id_proc
+          end
+          super(subclass)
+        end
+
         def self.root(plural, singular = nil, for_real = false)
           @type_plural = plural
           super(plural, singular) if for_real
         end
 
+        def self.entity_id(attribute_name=nil, &block)
+          self.custom_id_proc = if attribute_name != nil
+            ->(capture) { capture.send(attribute_name.to_sym) }
+          elsif block_given?
+            block
+          else
+            raise ArgumentError.new('must provide an attribute name or a block')
+          end
+        end
+
         expose :id, format_with: :to_string
         expose :json_api_type, as: :type
 
@@ -24,6 +41,10 @@ def self.root(plural, singular = nil, for_real = false)
 
         private
 
+        def id
+          self.class.custom_id_proc.is_a?(Proc) ? self.class.custom_id_proc.call(object) : object.id
+        end
+
         # rubocop:disable Metrics/AbcSize, Metrics/CyclomaticComplexity
 
         def json_api_type

spec/grape/jsonapi/entity/resource_identifier_spec.rb

@@ -93,4 +93,50 @@ def json_api_type
       end
     end
   end
+
+  context '#custom_id_proc' do
+    subject do
+      fresh_class.represent(data).serializable_hash
+    end
+
+    let(:record_id) { 'ID' }
+    let(:alternate_id_attribute) { 'ALT_ID_A' }
+    let(:alternate_id_block) { 'ALT_ID_B' }
+    let(:data) do
+      OpenStruct.new(
+        id: record_id,
+        other_attribute: alternate_id_attribute,
+      )
+    end
+
+    context 'when custom id is specified' do
+      let(:fresh_class) { Cow }
+
+      context 'as an attribute name' do
+        before { fresh_class.entity_id :other_attribute }
+
+        it 'uses the custom id' do
+          expect(subject[:id]).to eq alternate_id_attribute
+        end
+      end
+
+      context 'as a block' do
+        before { fresh_class.entity_id { |obj| alternate_id_block } }
+
+        it 'uses the custom id' do
+          expect(subject[:id]).to eq alternate_id_block
+        end
+      end
+
+    end
+
+    context 'when custom id is not specified' do
+      let(:fresh_class) { Mouse }
+
+      it 'uses the record id' do
+        expect(subject[:id]).to eq record_id
+      end
+    end
+
+  end
 end