Fix pseudo n+1 queries with Note and Note Authors in issuables APIs

This was not a clear cut n+1 query, given that if you're directly subscribed to all issues
that the API is returning you never really need to check for the notes. However, if you're
subscribed to _all_ of them, then for each issuable you need to go once to `notes`, and
once to `users` (for the authors). By preemtively loading notes and authors, at worst you have
1 extra query, and at best you saved 2n extra queries. We also took advantage of this preloading
of notes when counting user notes.

1 files changed, 21 insertions, 3 deletions

diff --git a/app/models/concerns/issuable.rb b/app/models/concerns/issuable.rb
index 58e7557fdc6..0ccd3474b81 100644
--- a/app/models/concerns/issuable.rb
+++ b/app/models/concerns/issuable.rb
@@ -17,7 +17,12 @@ module Issuable
     belongs_to :assignee, class_name: "User"
     belongs_to :updated_by, class_name: "User"
     belongs_to :milestone
-    has_many :notes, as: :noteable, dependent: :destroy
+    has_many :notes, as: :noteable, dependent: :destroy do
+      def authors_loaded?
+        # We check first if we're loaded to not load unnecesarily.
+        loaded? && to_a.all? { |note| note.association(:author).loaded? }
+      end
+    end
     has_many :label_links, as: :target, dependent: :destroy
     has_many :labels, through: :label_links
     has_many :todos, as: :target, dependent: :destroy
@@ -44,6 +49,7 @@ module Issuable
 
     scope :without_label, -> { joins("LEFT OUTER JOIN label_links ON label_links.target_type = '#{name}' AND label_links.target_id = #{table_name}.id").where(label_links: { id: nil }) }
     scope :join_project, -> { joins(:project) }
+    scope :inc_notes_with_associations, -> { includes(notes: :author) }
     scope :references_project, -> { references(:project) }
     scope :non_archived, -> { join_project.where(projects: { archived: false }) }
 
@@ -179,7 +185,13 @@ module Issuable
   end
 
   def user_notes_count
-    notes.user.count
+    if notes.loaded?
+      # Use the in-memory association to select and count to avoid hitting the db
+      notes.to_a.count { |note| !note.system? }
+    else
+      # do the count query
+      notes.user.count
+    end
   end
 
   def subscribed_without_subscriptions?(user)
@@ -239,7 +251,13 @@ module Issuable
   end
 
   def notes_with_associations
-    notes.includes(:author)
+    # If A has_many Bs, and B has_many Cs, and you do
+    # `A.includes(b: :c).each { |a| a.b.includes(:c) }`, sadly ActiveRecord
+    # will do the inclusion again. So, we check if all notes in the relation
+    # already have their authors loaded (possibly because the scope
+    # `inc_notes_with_associations` was used) and skip the inclusion if that's
+    # the case.
+    notes.authors_loaded? ? notes : notes.includes(:author)
   end
 
   def updated_tasks