Propagate nested extra_fields to links

Propagates the `extra_fields` attributes for a nested resource to the
relationship links created.

e.g. `GET http://example.com/books/1?extra_fields[authors]=biography`
should return a link to the `author` looking like
`http://example.com/authors/2?extra_fields[authors]=biography`,
propagating the `extra_fields` that apply for the nexted `authors`
resource.

Closes #328

Author: CGA1123

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Unreleased
 
 Features:
+- [329](https://github.com/graphiti-api/graphiti/pull/329) Propagate `extra_fields` to related resource links.
 - [242](https://github.com/graphiti-api/graphiti/pull/242) Bump `jsonapi-renderer` to `~0.2.2` now that (https://github.com/jsonapi-rb/jsonapi-renderer/pull/36) is fixed.
 - [158](https://github.com/graphiti-api/graphiti/pull/158) Filters options `allow_nil: true`
   Option can be set at the resource level `Resource.filters_accept_nil_by_default = true`. 

lib/graphiti/sideload.rb

@@ -136,6 +136,14 @@ def link_filter(parents)
       base_filter(parents)
     end
 
+    def link_extra_fields
+      extra_fields_name = [association_name, resource.type].find { |param|
+        context.params.dig(:extra_fields, param)
+      }
+
+      {resource.type => context.params.dig(:extra_fields, extra_fields_name)} if extra_fields_name
+    end
+
     # The parent resource is a remote,
     # AND the sideload is a remote to the same endpoint
     def shared_remote?

lib/graphiti/util/link.rb

@@ -63,6 +63,10 @@ def params
             params[:filter] = @sideload.link_filter([@model])
           end
 
+          if (extra_fields = @sideload.link_extra_fields)
+            params[:extra_fields] ||= extra_fields
+          end
+
           @sideload.params_proc&.call(params, [@model], context)
         end
       end

spec/serialization_spec.rb

@@ -1322,6 +1322,66 @@ def positions
           end
         end
 
+        context "with nested extra attribute" do
+          context "when referencing the type" do
+            let!(:employee) { PORO::Employee.create(classification_id: 789) }
+
+            before do
+              resource.belongs_to :classification
+              params[:extra_fields] = {classifications: "description"}
+            end
+
+            def classification
+              json["data"][0]["relationships"]["classification"]
+            end
+
+            it "links correctly" do
+              render
+              expect(classification["links"]["related"])
+                .to eq("/poro/classifications/789?extra_fields[classifications]=description")
+            end
+          end
+
+          context "when referencing the association name" do
+            before do
+              resource.has_many :positions, as: :senior_positions
+              params[:extra_fields] = {senior_positions: "score"}
+            end
+
+            it "links correctly" do
+              render
+              expect(positions["links"]["related"])
+                .to eq("/poro/positions?extra_fields[positions]=score&filter[employee_id]=1")
+            end
+          end
+
+          context "when referencing a polymorphic_belongs_to" do
+            let!(:employee) { PORO::Employee.create(credit_card_id: 1123, credit_card_type: "Visa") }
+
+            before do
+              resource.polymorphic_belongs_to :credit_card do
+                group_by(:credit_card_type) do
+                  on(:Visa)
+                  on(:GoldVisa)
+                  on(:Mastercard)
+                end
+              end
+
+              params[:extra_fields] = {credit_card: "foo"}
+            end
+
+            def credit_card
+              json["data"][0]["relationships"]["credit_card"]
+            end
+
+            it "links correctly" do
+              render
+              expect(credit_card["links"]["related"])
+                .to eq("/poro/visas/1123?extra_fields[visas]=foo")
+            end
+          end
+        end
+
         context "with runtime params" do
           xit "links correctly" do
           end