postgres aggregate string`

string_agg.rb

User.
  select("users.id, users.full_name, string_agg(roles.name, ', ') as roles_col").
  joins(:memberships => :role).
  group('users.id').
  each_with_object([[:id, :full_name, :roles]]){ |u,a| 
    a << [u.id, u.full_name, u.roles_col.split(',').map(&:strip) ]
  }

string_agg.sql

SELECT
  users.full_name, 
  string_agg(roles.name, ', ')
FROM
  users 
JOIN 
  memberships on memberships.user_id = users.id
JOIN
  roles on roles.id = memberships.role_id;