Address review comments

app/controllers/concerns/renders_notes.rb

+module RendersNotes
+  def prepare_notes_for_rendering(notes)
+    preload_noteable_for_regular_notes(notes)
+    preload_max_access_for_authors(notes, @project)
+    Banzai::NoteRenderer.render(notes, @project, current_user)
+
+    notes
+  end
+
+  private
+
+  def preload_max_access_for_authors(notes, project)
+    user_ids = notes.map(&:author_id)
+    project.team.max_member_access_for_user_ids(user_ids)
+  end
+
+  def preload_noteable_for_regular_notes(notes)
+    ActiveRecord::Associations::Preloader.new.preload(notes.reject(&:for_commit?), :noteable)
+  end
+end

app/controllers/projects/commit_controller.rb

@@ -2,7 +2,7 @@
 #
 # Not to be confused with CommitsController, plural.
 class Projects::CommitController < Projects::ApplicationController
-  include NotesHelper
+  include RendersNotes
   include CreatesCommit
   include DiffForPath
   include DiffHelper

app/controllers/projects/issues_controller.rb

 class Projects::IssuesController < Projects::ApplicationController
-  include NotesHelper
+  include RendersNotes
   include ToggleSubscriptionAction
   include IssuableActions
   include ToggleAwardEmoji

app/controllers/projects/merge_requests_controller.rb

@@ -3,7 +3,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   include DiffForPath
   include DiffHelper
   include IssuableActions
-  include NotesHelper
+  include RendersNotes
   include ToggleAwardEmoji
   include IssuableCollections

app/controllers/projects/notes_controller.rb

 class Projects::NotesController < Projects::ApplicationController
-  include NotesHelper
+  include RendersNotes
   include ToggleAwardEmoji
   # Authorize

app/controllers/projects/snippets_controller.rb

 class Projects::SnippetsController < Projects::ApplicationController
-  include NotesHelper
+  include RendersNotes
   include ToggleAwardEmoji
   include SpammableActions
   include SnippetsActions

app/finders/notes_finder.rb

@@ -20,9 +20,9 @@ class NotesFinder
   end
   def execute
-    @notes = init_collection
-    @notes = since_fetch_at(@params[:last_fetched_at], @notes) if @params[:last_fetched_at]
-    @notes
+    notes = init_collection
+    notes = since_fetch_at(notes)
+    notes.fresh
   end
   def target
@@ -56,7 +56,7 @@ class NotesFinder
   def notes_of_any_type
     types = %w(commit issue merge_request snippet)
     note_relations = types.map { |t| notes_for_type(t) }
-    note_relations.map! { |notes| search(@params[:search], notes) } if @params[:search]
+    note_relations.map! { |notes| search(notes) }
     UnionFinder.new.find_union(note_relations, Note)
   end
@@ -98,16 +98,21 @@ class NotesFinder
   #
   # This method uses ILIKE on PostgreSQL and LIKE on MySQL.
   #
-  def search(query, notes_relation = @notes)
+  def search(query, notes)
+    query = @params[:search]
+    return unless query
+
     pattern = "%#{query}%"
-    notes_relation.where(Note.arel_table[:note].matches(pattern))
+    notes.where(Note.arel_table[:note].matches(pattern))
   end
   # Notes changed since last fetch
   # Uses overlapping intervals to avoid worrying about race conditions
-  def since_fetch_at(fetch_time, notes_relation = @notes)
+  def since_fetch_at(notes)
+    return notes unless @params[:last_fetched_at]
+
     # Default to 0 to remain compatible with old clients
     last_fetched_at = Time.at(@params.fetch(:last_fetched_at, 0).to_i)
-    notes_relation.where('updated_at > ?', last_fetched_at - FETCH_OVERLAP).fresh
+    notes.updated_after(last_fetched_at - FETCH_OVERLAP)
   end
 end

app/helpers/notes_helper.rb

@@ -57,23 +57,6 @@ module NotesHelper
                            data: data, title: 'Add a reply'
   end
-  def preload_max_access_for_authors(notes, project)
-    user_ids = notes.map(&:author_id)
-    project.team.max_member_access_for_user_ids(user_ids)
-  end
-
-  def preload_noteable_for_regular_notes(notes)
-    ActiveRecord::Associations::Preloader.new.preload(notes.reject(&:for_commit?), :noteable)
-  end
-
-  def prepare_notes_for_rendering(notes)
-    preload_noteable_for_regular_notes(notes)
-    preload_max_access_for_authors(notes, @project)
-    Banzai::NoteRenderer.render(notes, @project, current_user)
-
-    notes
-  end
-
   def note_max_access_for_user(note)
     note.project.team.human_max_access(note.author_id)
   end

app/mailers/emails/notes.rb

@@ -4,7 +4,6 @@ module Emails
       setup_note_mail(note_id, recipient_id)
       @commit = @note.noteable
-      @discussion = @note.discussion if @note.part_of_discussion?
       @target_url = namespace_project_commit_url(*note_target_url_options)
       mail_answer_thread(@commit,
@@ -17,7 +16,6 @@ module Emails
       setup_note_mail(note_id, recipient_id)
       @issue = @note.noteable
-      @discussion = @note.discussion if @note.part_of_discussion?
       @target_url = namespace_project_issue_url(*note_target_url_options)
       mail_answer_thread(@issue, note_thread_options(recipient_id))
     end
@@ -26,7 +24,6 @@ module Emails
       setup_note_mail(note_id, recipient_id)
       @merge_request = @note.noteable
-      @discussion = @note.discussion if @note.part_of_discussion?
       @target_url = namespace_project_merge_request_url(*note_target_url_options)
       mail_answer_thread(@merge_request, note_thread_options(recipient_id))
     end
@@ -35,7 +32,6 @@ module Emails
       setup_note_mail(note_id, recipient_id)
       @snippet = @note.noteable
-      @discussion = @note.discussion if @note.part_of_discussion?
       @target_url = namespace_project_snippet_url(*note_target_url_options)
       mail_answer_thread(@snippet, note_thread_options(recipient_id))
     end
@@ -44,7 +40,6 @@ module Emails
       setup_note_mail(note_id, recipient_id)
       @snippet = @note.noteable
-      @discussion = @note.discussion if @note.part_of_discussion?
       @target_url = snippet_url(@note.noteable)
       mail_answer_thread(@snippet, note_thread_options(recipient_id))
     end

app/models/concerns/resolvable_discussion.rb

@@ -2,12 +2,14 @@ module ResolvableDiscussion
   extend ActiveSupport::Concern
   included do
-    memoized_values << :resolvable
-    memoized_values << :resolved
-    memoized_values << :first_note
-    memoized_values << :first_note_to_resolve
-    memoized_values << :last_resolved_note
-    memoized_values << :last_note
+    memoized_values.push(
+      :resolvable,
+      :resolved,
+      :first_note,
+      :first_note_to_resolve,
+      :last_resolved_note,
+      :last_note
+    )
     delegate  :resolved_at,
               :resolved_by,

app/models/concerns/resolvable_note.rb

@@ -8,7 +8,9 @@ module ResolvableNote
     validates :resolved_by, presence: true, if: :resolved?
-    # Keep this scope in sync with the logic in `#potentially_resolvable?` in `Discussion` subclasses that are resolvable
+    # Keep this scope in sync with the logic in `#potentially_resolvable?` in `Discussion` subclasses that are resolvable.
+    # `RESOLVABLE_TYPES` should include names of all subclasses that are resolvable (where the method can return true), and
+    # the scope should also match the criteria `ResolvableDiscussion#potentially_resolvable?` puts on resolvability.
     scope :potentially_resolvable, -> { where(type: RESOLVABLE_TYPES).where(noteable_type: 'MergeRequest') }
     # Keep this scope in sync with `#resolvable?`
     scope :resolvable, -> { potentially_resolvable.user }

app/models/diff_discussion.rb

+# A discussion on merge request or commit diffs consisting of `DiffNote` notes
 class DiffDiscussion < Discussion
   include DiscussionOnDiff

app/models/diff_note.rb

+# A note on merge request or commit diffs
 class DiffNote < Note
   include NoteOnDiff

app/models/discussion.rb

@@ -39,6 +39,7 @@ class Discussion
     [:discussion, note.noteable_type.try(:underscore), noteable_id]
   end
+  # Returns an array of discussion ID components
   def self.build_discussion_id(note)
     [*build_discussion_id_base(note), SecureRandom.hex]
   end

app/models/discussion_note.rb

+# A note in a non-diff discussion on an issue, merge request, commit, or snippet
 class DiscussionNote < Note
   NOTEABLE_TYPES = %w(MergeRequest Issue Commit Snippet).freeze

app/models/individual_note_discussion.rb

+# A discussion to wrap a single `Note` note on the root of an issue, merge request,
+# commit, or snippet, that is not displayed as a discussion
 class IndividualNoteDiscussion < Discussion
   # Keep this method in sync with the `potentially_resolvable` scope on `ResolvableNote`
   def potentially_resolvable?

app/models/legacy_diff_discussion.rb

+# A discussion on merge request or commit diffs consisting of `LegacyDiffNote` notes
 class LegacyDiffDiscussion < Discussion
   include DiscussionOnDiff

app/models/legacy_diff_note.rb

+# A note on merge request or commit diffs, using the legacy implementation
 class LegacyDiffNote < Note
   include NoteOnDiff

app/models/note.rb

@@ -68,6 +68,7 @@ class Note < ActiveRecord::Base
   scope :user, ->{ where(system: false) }
   scope :common, ->{ where(noteable_type: ["", nil]) }
   scope :fresh, ->{ order(created_at: :asc, id: :asc) }
+  scope :updated_after, ->(time){ where('updated_at > ?', time) }
   scope :inc_author_project, ->{ includes(:project, :author) }
   scope :inc_author, ->{ includes(:author) }
   scope :inc_relations_for_view, -> do
@@ -238,18 +239,20 @@ class Note < ActiveRecord::Base
     discussion_class(noteable).override_discussion_id(self) || super()
   end
-  # Returns a discussion containing just this note
+  # Returns a discussion containing just this note.
+  # This method exists as an alternative to `#discussion` to use when the methods
+  # we intend to call on the Discussion object don't require it to have all of its notes,
+  # and just depend on the first note or the type of discussion. This saves us a DB query.
   def to_discussion(noteable = nil)
     Discussion.build([self], noteable)
   end
-  # Returns the entire discussion this note is part of
+  # Returns the entire discussion this note is part of.
+  # Consider using `#to_discussion` if we do not need to render the discussion
+  # and all its notes and if we don't care about the discussion's resolvability status.
   def discussion
-    if part_of_discussion?
-      self.noteable.notes.find_discussion(self.discussion_id) || to_discussion
-    else
-      to_discussion
-    end
+    full_discussion = self.noteable.notes.find_discussion(self.discussion_id) if part_of_discussion?
+    full_discussion || to_discussion
   end
   def part_of_discussion?

app/models/out_of_context_discussion.rb

+# A discussion to wrap a number of `Note` notes on the root of a commit when they
+# are displayed in context of a merge request as if they were part of a discussion.
 class OutOfContextDiscussion < Discussion
   # To make sure all out-of-context notes are displayed in one discussion,
   # we override the discussion ID to be a newly generated but consistent ID.