Add support for reverse link set links in GraphQL

This adds support for reverse links that are part of a link set.

app/graphql/sources/reverse_linked_to_editions_source.rb

@@ -0,0 +1,29 @@
+module Sources
+  class ReverseLinkedToEditionsSource < GraphQL::Dataloader::Source
+    # rubocop:disable Lint/MissingSuper
+    def initialize(parent_object:)
+      @object = parent_object
+      @content_store = parent_object.content_store.to_sym
+    end
+    # rubocop:enable Lint/MissingSuper
+
+    def fetch(link_types)
+      all_links = @object
+        .document
+        .reverse_links
+        .where(link_type: link_types)
+
+      link_types_map = link_types.index_with { [] }
+
+      all_links.each_with_object(link_types_map) { |link, hash|
+        hash[link.link_type].concat(editions_for_link(link))
+      }.values
+    end
+
+  private
+
+    def editions_for_link(link)
+      link.source_documents.map { |document| document.send(@content_store) }
+    end
+  end
+end

app/graphql/types/base_object.rb

@@ -14,5 +14,14 @@ def self.links_field(field_name_and_link_type, graphql_field_type)
           .load(field_name_and_link_type.to_s)
       end
     end
+
+    def self.reverse_links_field(field_name_and_link_type, belongs_to, graphql_field_type)
+      field(field_name_and_link_type.to_sym, graphql_field_type)
+
+      define_method(field_name_and_link_type.to_sym) do
+        dataloader.with(Sources::ReverseLinkedToEditionsSource, parent_object: object)
+          .load(belongs_to.to_s)
+      end
+    end
   end
 end

app/graphql/types/edition_type.rb

@@ -89,6 +89,17 @@ class EditionLinks < Types::BaseObject
       links_field :worldwide_organisation, [EditionType]
       links_field :worldwide_organisations, [EditionType]
 
+      reverse_links_field :child_taxons, :parent_taxons, [EditionType]
+      reverse_links_field :children, :parent, [EditionType]
+      reverse_links_field :document_collections, :documents, [EditionType]
+      reverse_links_field :level_one_taxons, :root_taxon, [EditionType]
+      reverse_links_field :ministers, :ministerial, [EditionType]
+      reverse_links_field :part_of_step_navs, :pages_part_of_step_nav, [EditionType]
+      reverse_links_field :policies, :working_groups, [EditionType]
+      reverse_links_field :related_to_step_navs, :pages_related_to_step_nav, [EditionType]
+      reverse_links_field :role_appointments, :person, [EditionType]
+      reverse_links_field :secondary_to_step_navs, :secondary_to_step_navs, [EditionType]
+
       def available_translations
         Presenters::Queries::AvailableTranslations.by_edition(object)
           .translations.fetch(:available_translations, [])

spec/graphql/sources/reverse_linked_to_editions_source_spec.rb

@@ -0,0 +1,20 @@
+RSpec.describe Sources::ReverseLinkedToEditionsSource do
+  it "returns the specified reverse link set links" do
+    target_document = create(:edition)
+    source_document_1 = create(:edition)
+    source_document_2 = create(:edition)
+    source_document_3 = create(:edition)
+    link_set_1 = create(:link_set, content_id: source_document_1.content_id)
+    link_set_2 = create(:link_set, content_id: source_document_2.content_id)
+    link_set_3 = create(:link_set, content_id: source_document_3.content_id)
+    create(:link, link_set: link_set_1, target_content_id: target_document.content_id, link_type: "test_link")
+    create(:link, link_set: link_set_2, target_content_id: target_document.content_id, link_type: "another_link_type")
+    create(:link, link_set: link_set_3, target_content_id: target_document.content_id, link_type: "test_link")
+
+    GraphQL::Dataloader.with_dataloading do |dataloader|
+      request = dataloader.with(described_class, parent_object: target_document).request("test_link")
+
+      expect(request.load).to eq([source_document_1, source_document_3])
+    end
+  end
+end