Fix including with blank relationships

Author: stokarenko

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- [#350](https://github.com/JsonApiClient/json_api_client/pull/350) - fix resource including with blank `relationships` response data
+
 ## 1.12.1
 
 - [#349](https://github.com/JsonApiClient/json_api_client/pull/349) - fix resource including for STI objects

lib/json_api_client/resource.rb

@@ -546,8 +546,7 @@ def relationship_data_for(name, relationship_definition)
         end
       end
 
-      url = relationship_definition["links"]["related"]
-      if relationship_definition["links"] && url
+      if url = relationship_definition.dig("links", "related")
         return association_for(name).data(url)
       end
 

test/unit/association_test.rb

@@ -896,6 +896,28 @@ def test_get_with_type_attribute
     assert user.file.is_a?(DocumentFile)
   end
 
+  def test_include_with_blank_relationships
+    stub_request(:get, "http://example.com/document_users/1?include=file")
+        .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+            data: [
+                {
+                    id: '1',
+                    type: 'document_users',
+                    attributes: {
+                        name: 'John Doe'
+                    },
+                    relationships: {
+                        file: {
+                        }
+                    }
+                }
+            ],
+        }.to_json)
+
+    user = DocumentUser.includes('file').find(1).first
+    assert_nil user.file
+  end
+
   def test_load_include_from_dataset
     stub_request(:get, 'http://example.com/employees?include=chief&page[per_page]=2')
         .to_return(