How to promote a regular user in omnibus-gitlab to admin

Launch gitlab-rails console and set user.admin to true.
$ sudo gitlab-rails console
Loading production environment (Rails 4.1.1)
irb(main):014:0> user = User.find_by(name:'XXXXXX')
irb(main):015:0> user.admin="true"
=> "true"
irb(main):016:0> user.save
=> true