Only SELECT person's roles columns that we need

The idea with this change is to reduce the amount of data retrieved from
the database in an attempt to speed up the overall response time to the
GraphQL client.

So far we've implemented this optimisation for the root edition,
linked-to editions and reverse-linked editions. Here we're introducing
the change for the person's role_appointments/roles fields specific to
the ministers index type.

We've followed the patterns established in the previous implementations
with the big differences being mostly:
* the larger number of JOINs needed in this query because we're
  traversing 2 levels of links
* the convenience of only needing to handle link set links here because
  we know that those are the only types of links involved in linking the
  ministers index's person-role_appointments-roles

Because of the large number of JOINS, we've resorted to supplying SQL
strings in order to provide our own, hopefully clearer, join table
aliases as opposed to leaving that to Active Record to generate.

app/graphql/sources/person_current_roles_source.rb

@@ -1,51 +1,78 @@
 module Sources
   class PersonCurrentRolesSource < GraphQL::Dataloader::Source
-    def fetch(person_content_ids)
+    # rubocop:disable Lint/MissingSuper
+    def initialize
+      @content_store = :live
+    end
+    # rubocop:enable Lint/MissingSuper
+
+    def fetch(person_content_ids_and_selections)
+      person_content_ids = []
+      ids_map = {}
+      all_selections = {
+        role_appointment_links: %i[target_content_id],
+        documents: %i[content_id],
+      }
+      editions_selections = Set.new
+
+      person_content_ids_and_selections.each do |person_content_id, selections|
+        person_content_ids << person_content_id
+        ids_map[person_content_id] = []
+        editions_selections.merge(selections)
+      end
+
+      all_selections[:editions] = editions_selections.to_a
+
       all_roles = Edition
-        .live
-        .includes(
-          document: {
-            reverse_links: { # role -> role_appointment
-              link_set: {
-                documents: [
-                  :editions, # role_appointment
-                  :link_set_links, # role_appointment -> person
-                ],
-              },
-            },
-          },
+        .joins(document: [reverse_links: :link_set])
+        .joins(
+          <<~SQL,
+            INNER JOIN documents role_appointment_documents
+            ON role_appointment_documents.content_id = link_sets.content_id
+          SQL
+        )
+        .joins(
+          <<~SQL,
+            INNER JOIN editions role_appointment_editions
+            ON role_appointment_editions.document_id = role_appointment_documents.id
+          SQL
+        )
+        .joins(
+          <<~SQL,
+            INNER JOIN link_sets role_appointment_link_sets
+            ON role_appointment_link_sets.content_id = role_appointment_documents.content_id
+          SQL
+        )
+        .joins(
+          <<~SQL,
+            INNER JOIN links role_appointment_links
+            ON role_appointment_links.link_set_id = role_appointment_link_sets.id
+          SQL
         )
         .where(
-          document_type: "ministerial_role",
-          document: { locale: "en" },
+          editions: {
+            content_store: @content_store,
+            document_type: "ministerial_role",
+          },
+          documents: { locale: "en" },
           reverse_links: { link_type: "role" },
-          documents: { locale: "en" }, # role_appointment Documents
-          editions_documents: { document_type: "role_appointment" }, # role_appointment Editions
-          link_set_links: { target_content_id: person_content_ids, link_type: "person" },
+          role_appointment_documents: { locale: "en" },
+          role_appointment_editions: {
+            content_store: @content_store,
+            document_type: "role_appointment",
+          },
+          role_appointment_links: {
+            target_content_id: person_content_ids,
+            link_type: "person",
+          },
         )
-        .where("editions_documents.details ->> 'current' = 'true'") # editions_documents is the alias that Active Record gives to the role_appointment Editions in the SQL query
+        .where("role_appointment_editions.details ->> 'current' = 'true'")
         .order(reverse_links: { position: :asc })
-
-      ids_map = person_content_ids.index_with { [] }
+        .select(all_selections)
 
       all_roles.each_with_object(ids_map) { |role, hash|
-        hash[person_content_id_for_role(role)] << role
+        hash[role.target_content_id] << role
       }.values
     end
-
-  private
-
-    def person_content_id_for_role(role)
-      role_appointment_documents_for_role(role)
-        .flat_map(&:link_set_links)
-        .find { |link| link.link_type == "person" } # role_appointment -> person
-        .target_content_id
-    end
-
-    def role_appointment_documents_for_role(role)
-      role.document.reverse_links
-        .select { |link| link.link_type == "role" } # role -> role_appointment
-        .flat_map { |link| link.link_set.documents }
-    end
   end
 end

app/graphql/types/ministers_index_type.rb

@@ -51,11 +51,16 @@ class MinistersIndexPersonDetails < Types::BaseObject
       end
 
       class MinistersIndexPersonLinks < Types::BaseObject
-        field :role_appointments, [MinistersIndexRoleAppointment]
+        field :role_appointments, [MinistersIndexRoleAppointment], extras: [:lookahead]
+
+        def role_appointments(lookahead:)
+          links_lookahead = lookahead.selections.find { _1.name == :links }
+          role_lookahead = links_lookahead.selections.find { _1.name == :role }
+
+          selections = convert_edition_selections(lookahead: role_lookahead)
 
-        def role_appointments
           dataloader.with(Sources::PersonCurrentRolesSource)
-            .load(object.content_id)
+            .load([object.content_id, selections])
         end
       end
 

spec/graphql/sources/person_current_roles_source_spec.rb

@@ -11,7 +11,10 @@
     appoint_person_to_role(person_1, role_3)
 
     GraphQL::Dataloader.with_dataloading do |dataloader|
-      request = dataloader.with(described_class).request(person_1.content_id)
+      request = dataloader.with(described_class).request([
+        person_1.content_id,
+        %i[id base_path title details],
+      ])
 
       expect(request.load).to eq([role_1, role_3])
     end