Linq query to get a list of roles for a user in identity framework using blazor c#

using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Identity;

I want to get a list with UserName, UserId, and a string with all the roles that the user is assigned to. 

Basically, I want to return  "User's Id", "UserName", "Admin, User, Guest" (roles)

What I have so far is shown below, Just don't know how to do the ????? to get the list.

_db is just public class ApplicationDbContext : IdentityDbContext<IdentityUser>

            var users = from u in _db.Users
                join rr in _db.UserRoles on u.Id equals rr.UserId
                join r in _db.Roles on rr.RoleId equals r.Id
                select new UserWithRoles { UserName = u.UserName,
                    UserId = u.Id,
                    AssignedRoles = String.Join(", ",??????) };

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