Exclude id in serialized payload when nil.

lib/jsonapi-serializers/serializer.rb

@@ -359,10 +359,15 @@ def self.serialize_primary(object, options = {})
 
       serializer = serializer_class.new(object, options)
       data = {
-        'id' => serializer.id.to_s,
         'type' => serializer.type.to_s,
       }
 
+      # "The id member is not required when the resource object originates at the client
+      #  and represents a new resource to be created on the server."
+      # http://jsonapi.org/format/#document-resource-objects
+      # We'll assume that if the id is blank, it means the resource is to be created.
+      data['id'] = serializer.id.to_s if serializer.id && !serializer.id.empty?
+
       # Merge in optional top-level members if they are non-nil.
       # http://jsonapi.org/format/#document-structure-resource-objects
       # Call the methods once now to avoid calling them twice when evaluating the if's below.

spec/serializer_spec.rb

@@ -129,6 +129,22 @@ def serialize_primary(object, options = {})
           },
         })
       end
+      it 'does not include id when it is nil' do
+        post = create(:post)
+        post.id = nil
+        primary_data = serialize_primary(post, {serializer: MyApp::PostSerializerWithoutLinks})
+        expect(primary_data).to eq({
+          'type' => 'posts',
+          'attributes' => {
+            'title' => 'Title for Post 1',
+            'long-content' => 'Body for Post 1',
+          },
+          'relationships' => {
+            'author' => {},
+            'long-comments' => {},
+          },
+        })
+      end
       it 'serializes object when multiple attributes are declared once' do
         post = create(:post)
         primary_data = serialize_primary(post, {serializer: MyApp::MultipleAttributesSerializer})