Refactor plain SQL to active record relations 🚀

app/models/issue.rb

@@ -35,6 +35,12 @@ class Issue < ActiveRecord::Base
   has_many :issue_assignees
   has_many :assignees, class_name: "User", through: :issue_assignees
 
+  has_many :referenced_issue_links, class_name: 'IssueLink', foreign_key: :source_id
+  has_many :referred_by_issue_links, class_name: 'IssueLink', foreign_key: :target_id
+
+  has_many :referenced_issues, through: :referenced_issue_links, source: :target
+  has_many :referred_by_issues, through: :referred_by_issue_links, source: :source
+
   validates :project, presence: true
 
   scope :in_projects, ->(project_ids) { where(project_id: project_ids) }

app/services/issue_links/list_service.rb

@@ -26,21 +26,13 @@ module IssueLinks
     def issues
       return @issues if defined?(@issues)
 
-      # TODO: Simplify query using AR
-      @issues = Issue.find_by_sql(
-        <<-SQL.strip_heredoc
-          SELECT issues.*, issue_links.id as issue_links_id FROM issues
-          INNER JOIN issue_links ON issue_links.target_id = issues.id
-          WHERE issue_links.source_id = #{@issue.id} AND issues.deleted_at IS NULL
-          UNION ALL
-          SELECT issues.*, issue_links.id as issue_links_id FROM issues
-          INNER JOIN issue_links ON issue_links.source_id = issues.id
-          WHERE issue_links.target_id = #{@issue.id} AND issues.deleted_at IS NULL
-          ORDER BY issue_links_id
-        SQL
-      )
+      referenced_issues = @issue.referenced_issues.select('issues.*', 'issue_links.id AS issue_links_id')
+      referred_by_issues = @issue.referred_by_issues.select('issues.*', 'issue_links.id AS issue_links_id')
+      union = Gitlab::SQL::Union.new([referenced_issues, referred_by_issues])
 
-      # TODO: Try to use SQL instead Array#select
+      sql = "#{union.to_sql} ORDER BY issue_links_id"
+
+      @issues = Issue.find_by_sql(sql)
       @issues = Ability.issues_readable_by_user(@issues, @current_user)
     end