diff --git a/CHANGELOG.md b/CHANGELOG.md
index 23016f2a..314daa3e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- [#349](https://github.com/JsonApiClient/json_api_client/pull/349) - fix resource including for STI objects
+
 ## 1.12.0
 
 - [#345](https://github.com/JsonApiClient/json_api_client/pull/345) - track the real HTTP reason of ApiErrors
diff --git a/lib/json_api_client/included_data.rb b/lib/json_api_client/included_data.rb
index c62e1f73..0b7063f4 100644
--- a/lib/json_api_client/included_data.rb
+++ b/lib/json_api_client/included_data.rb
@@ -4,20 +4,30 @@ class IncludedData
 
     def initialize(result_set, data)
       record_class = result_set.record_class
-      included_set = data.map do |datum|
-        type = datum["type"]
+      grouped_data = data.group_by{|datum| datum["type"]}
+      grouped_included_set = grouped_data.each_with_object({}) do |(type, records), h|
         klass = Utils.compute_type(record_class, record_class.key_formatter.unformat(type).singularize.classify)
-        params = klass.parser.parameters_from_resource(datum)
-        resource = klass.load(params)
-        resource.last_result_set = result_set
-        resource
+        h[type] = records.map do |record|
+          params = klass.parser.parameters_from_resource(record)
+          klass.load(params).tap do |resource|
+            resource.last_result_set = result_set
+          end
+        end
       end
 
-      included_set.concat(result_set) if record_class.search_included_in_result_set
-      @data = included_set.group_by(&:type).inject({}) do |h, (type, resources)|
-        h[type] = resources.index_by(&:id)
-        h
+      if record_class.search_included_in_result_set
+        # deep_merge overrides the nested Arrays o_O
+        # {a: [1,2]}.deep_merge(a: [3,4]) # => {a: [3,4]}
+        grouped_included_set.merge!(result_set.group_by(&:type)) do |_, resources1, resources2|
+          resources1 + resources2
+        end
       end
+
+      grouped_included_set.each do |type, resources|
+        grouped_included_set[type] = resources.index_by(&:id)
+      end
+
+      @data = grouped_included_set
     end
 
     def data_for(method_name, definition)
diff --git a/test/unit/association_test.rb b/test/unit/association_test.rb
index dcdd4216..65e86e1b 100644
--- a/test/unit/association_test.rb
+++ b/test/unit/association_test.rb
@@ -854,6 +854,48 @@ def test_get_with_defined_relationship_for_model_with_custom_type
     assert user.files.first.is_a?(DocumentFile)
   end
 
+  def test_get_with_type_attribute
+    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: {
+                            links: {
+                                self: 'http://example.com/document_users/1/relationships/file',
+                                related: 'http://example.com/document_users/1/file'
+                            },
+                            data: {
+                                id: '2',
+                                type: 'document--files'
+                            }
+                        }
+                    }
+                }
+            ],
+            included: [
+                {
+                    id: '2',
+                    type: 'document--files',
+                    attributes: {
+                        type: 'STIDocumentFile',
+                        url: 'http://example.com/downloads/2.pdf'
+                    }
+                }
+            ]
+        }.to_json)
+
+    user = DocumentUser.includes('file').find(1).first
+
+    assert_equal 'STIDocumentFile', user.file.type
+    assert user.file.is_a?(DocumentFile)
+  end
+
   def test_load_include_from_dataset
     stub_request(:get, 'http://example.com/employees?include=chief&page[per_page]=2')
         .to_return(