[Issue multiple assignee] Use sub-query instead of join

app/finders/issues_finder.rb

@@ -39,18 +39,15 @@ class IssuesFinder < IssuableFinder
   end
 
   def self.not_restricted_by_confidentiality(user)
-    issues = Issue.with_assignees
+    return Issue.where('issues.confidential IS NOT TRUE') if user.blank?
 
-    return issues.where('issues.confidential IS NULL OR issues.confidential IS FALSE') if user.blank?
+    return Issue.all if user.admin_or_auditor?
 
-    return issues.all if user.admin_or_auditor?
-
-    issues.where('
-      issues.confidential IS NULL
-      OR issues.confidential IS FALSE
+    Issue.where('
+      issues.confidential IS NOT TRUE
       OR (issues.confidential = TRUE
         AND (issues.author_id = :user_id
-          OR issue_assignees.user_id = :user_id
+          OR EXISTS (SELECT true FROM issue_assignees WHERE user_id = :user_id AND issue_id = issues.id)
           OR issues.project_id IN(:project_ids)))',
       user_id: user.id,
       project_ids: user.authorized_projects(Gitlab::Access::REPORTER).select(:id))

app/models/issue.rb

@@ -34,13 +34,11 @@ class Issue < ActiveRecord::Base
 
   validates :project, presence: true
 
-  scope :cared, ->(user) { with_assignees.where("issue_assignees.user_id IN(?)", user.id) }
   scope :open_for, ->(user) { opened.assigned_to(user) }
   scope :in_projects, ->(project_ids) { where(project_id: project_ids) }
-  scope :with_assignees, -> { joins("LEFT JOIN issue_assignees ON issue_id = issues.id") }
-  scope :assigned, -> { with_assignees.where('issue_assignees.user_id IS NOT NULL') }
-  scope :unassigned, -> { with_assignees.where('issue_assignees.user_id IS NULL') }
-  scope :assigned_to, ->(u) { with_assignees.where('issue_assignees.user_id = ?', u.id)}
+  scope :assigned, -> { where('EXISTS (SELECT * FROM issue_assignees WHERE issue_id = issues.id)') }
+  scope :unassigned, -> { where('NOT EXISTS (SELECT * FROM issue_assignees WHERE issue_id = issues.id)') }
+  scope :assigned_to, ->(u) { where('EXISTS (SELECT true FROM issue_assignees WHERE user_id = ? AND issue_id = issues.id)', u.id)}
 
   scope :without_due_date, -> { where(due_date: nil) }
   scope :due_before, ->(date) { where('issues.due_date < ?', date) }