rails 4 activerecord group duplicate entries and concat into a column

I'm relatively new to rails and I love ActiveRecord, but I am still getting used to generating ActiveRecord queries equivalent to what I would do in SQLite. I want to rely on ActiveRecord as much as possible since it will be easier to swap out the database in the future. I have the following tables:

Tables

projects
id | title | etc
---+-------+----
 1 | foo   | 

roles
id |   name  | etc
---+---------+-----
 1 | manager | 
 2 | editor  | 
 3 | author  | 

users
id | name | etc
---+------+-----
 1 | tom  | 
 2 | jerry|

team_memberships
id | project_id | member_id | role_id 
---+------------+-----------+---------
 1 |     1      |     1     |     1   
 2 |     1      |     1     |     2
 3 |     1      |     2     |     3

 *NOTE: member_id is a foreign key of the User model's id

In my Project model, I have a has_many association called team_memberships. A TeamMember can take on multiple Roles for a project. In such a case if I loop through the project's team_memberships (@project.team_memberships.each), I get duplicates members. I want to concat the roles into another column sorted by roles.name asc like so:

Desired output

 member_id | roles
-----------+-------------------
    2      |  author
    1      |  editor, manager 

I have been able to achieve this by using group_by and mapping it into a new array, but that is not grouping via ActiveRecord.

team_member_roles = project.team_memberships.select("roles.name").joins(:role).includes(:role, :member).order("roles.name").group_by(&:member_id).map do | key, memberships |
  { :member => memberships.first.member, :roles => memberships.map {|membership| membership.role.name }.join(",")  }
end

How would I accomplish this the ActiveRecord way? Would using group_by be scoffed at? Are there performance issues I should be concerned about? Like I said, I'm new to Rails and I want to make sure that my code not only just works but follows rails conventions.