diff --git a/lib/sinja.rb b/lib/sinja.rb
index 5d20636..e77245e 100644
--- a/lib/sinja.rb
+++ b/lib/sinja.rb
@@ -157,6 +157,15 @@ def data
         end
       end
 
+      def included
+        @included ||= {}
+        @included[request.path] ||= begin
+          deserialize_request_body.fetch(:included, nil)
+        rescue NoMethodError, KeyError
+          raise BadRequestError, 'Malformed {json:api} request payload'
+        end
+      end
+
       def normalize_filter_params
         return {} unless params[:filter]&.any?
 
diff --git a/lib/sinja/helpers/relationships.rb b/lib/sinja/helpers/relationships.rb
index e7579ca..90f9ef1 100644
--- a/lib/sinja/helpers/relationships.rb
+++ b/lib/sinja/helpers/relationships.rb
@@ -23,6 +23,7 @@ def dispatch_relationship_requests!(id, methods: {}, **opts)
         rels = data.fetch(:relationships, {}).to_a
         rels.each do |rel, body, rel_type=nil, count=0|
           rel_type ||= settings._resource_config[:has_one].key?(rel) ? :has_one : :has_many
+          body = body.merge(included: included) if included
           code, _, *json = dispatch_relationship_request id, rel,
             opts.merge(:body=>body, :method=>methods.fetch(rel_type, :patch))