Optimise GET api/v2/plans / Fix N+1 queries

This change further addresses Bullet warnings that were earlier addressed in commit 6ed969b

Author: aaronskiba

app/models/plan.rb

@@ -211,16 +211,34 @@ class Plan < ApplicationRecord
     includes(:phases, :sections, :questions, template: [:org]).find(id)
   }
 
+  # Eager loads all associations needed for API v2 serialization,
+  # and restricts to plans where the user_id has an active role.
   scope :for_api_v2, lambda { |user_id|
     joins(:roles)
       .includes(
-        :identifiers,
         :research_outputs,
         :template,
-        funder: :identifiers,
-        contributors: [:identifiers, { org: :identifiers }],
-        org: %i[region identifiers],
-        roles: [user: [:identifiers, { org: :identifiers }]]
+        { identifiers: :identifier_scheme },
+        funder: { identifiers: :identifier_scheme },
+        contributors: [
+          { identifiers: :identifier_scheme },
+          { org: { identifiers: :identifier_scheme } }
+        ],
+        roles: [
+          user: [
+            :language,
+            { identifiers: :identifier_scheme },
+            { org: { identifiers: :identifier_scheme } }
+          ]
+        ],
+        # plan.org is only executed when `plan.funder.present? || plan.grant_id.present? == true`
+        # - (see `app/views/api/v2/plans/_project.json.jbuilder`)
+        # Thus, the following line avoids N+1 queries in some cases,
+        # but performs unnecessary eager loading in others
+        org: [
+          :region,
+          { identifiers: :identifier_scheme }
+        ]
       )
       .where(roles: { user_id: user_id, active: true })
       .distinct