Significantly reduce N+1 queries in /api/v4/todos endpoint

By preloading associations and batching issuable metadata lookups,
we can significantly cut the number of SQL queries needed to load
the Todos API endpoint.

On GitLab.com, my own tests showed my user's SQL queries went
from 365 to under 60 SQL queries.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40378

4 files changed, 35 insertions, 4 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 5176e9713c1..2cd0d93b205 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -883,7 +883,8 @@ module API
       expose :target_type
 
       expose :target do |todo, options|
-        todo_target_class(todo.target_type).represent(todo.target, options)
+        todo_options = options.fetch(todo.target_type, {})
+        todo_target_class(todo.target_type).represent(todo.target, todo_options)
       end
 
       expose :target_url do |todo, options|
diff --git a/lib/api/issues.rb b/lib/api/issues.rb
index f43f4d961d6..7b8c070ad57 100644
--- a/lib/api/issues.rb
+++ b/lib/api/issues.rb
@@ -28,7 +28,7 @@ module API
         args[:scope] = args[:scope].underscore if args[:scope]
 
         issues = IssuesFinder.new(current_user, args).execute
-          .preload(:assignees, :labels, :notes, :timelogs, :project, :author, :closed_by)
+                   .with_api_entity_associations
         issues.reorder(order_options_with_tie_breaker)
       end
       # rubocop: enable CodeReuse/ActiveRecord
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 44f1e81caf2..28072bdf586 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -45,7 +45,7 @@ module API
         return merge_requests if args[:view] == 'simple'
 
         merge_requests
-          .preload(:notes, :author, :assignee, :milestone, :latest_merge_request_diff, :labels, :timelogs, metrics: [:latest_closed_by, :merged_by])
+          .with_api_entity_associations
       end
       # rubocop: enable CodeReuse/ActiveRecord
 
diff --git a/lib/api/todos.rb b/lib/api/todos.rb
index 64ac8ece56c..d2196f05173 100644
--- a/lib/api/todos.rb
+++ b/lib/api/todos.rb
@@ -6,6 +6,8 @@ module API
 
     before { authenticate! }
 
+    helpers ::Gitlab::IssuableMetadata
+
     ISSUABLE_TYPES = {
       'merge_requests' => ->(iid) { find_merge_request_with_access(iid) },
       'issues' => ->(iid) { find_project_issue(iid) }
@@ -42,6 +44,30 @@ module API
         def find_todos
           TodosFinder.new(current_user, params).execute
         end
+
+        def issuable_and_awardable?(type)
+          obj_type = Object.const_get(type)
+
+          (obj_type < Issuable) && (obj_type < Awardable)
+        rescue NameError
+          false
+        end
+
+        def batch_load_issuable_metadata(todos, options)
+          # This should be paginated and will cause Rails to SELECT for all the Todos
+          todos_by_type = todos.group_by(&:target_type)
+
+          todos_by_type.keys.each do |type|
+            next unless issuable_and_awardable?(type)
+
+            collection = todos_by_type[type]
+
+            next unless collection
+
+            targets = collection.map(&:target)
+            options[type] = { issuable_metadata: issuable_meta_data(targets, type) }
+          end
+        end
       end
 
       desc 'Get a todo list' do
@@ -51,7 +77,11 @@ module API
         use :pagination
       end
       get do
-        present paginate(find_todos), with: Entities::Todo, current_user: current_user
+        todos = paginate(find_todos.with_api_entity_associations)
+        options = { with: Entities::Todo, current_user: current_user }
+        batch_load_issuable_metadata(todos, options)
+
+        present todos, options
       end
 
       desc 'Mark a todo as done' do