Linq - Join collection with child collection of other collection - linq

Question

I have a collection of users, each with a child collection of memberships. And then I also have a club collection. The child collection of each user is populated with memberships containing a club ID only. What I want is for each membership to also contain the club entity matching the club ID.
My entities:
public class User
{
public string Name { get; set; }
public IEnumerable<Membership> Memberships { get; set; }
}
public class Membership
{
public Guid ClubId { get; set; }
public Club Club { get; set; }
}
public class Club
{
public Guid Id { get; set; }
public string Name { get; set; }
}
Populated with data:
var club1Id = new Guid("FFEB77B9-1616-463B-B36E-F95F7E255FDE");
var club2Id = new Guid("7A6ECD38-5AEB-418B-9DD5-FBD777CE190C");
var users = new List<User>
{
new User
{
Name = "Patrick Stewart",
Memberships = new List<Membership>
{
new Membership { ClubId = club1Id },
new Membership { ClubId = club2Id }
}
},
new User
{
Name = "Brent Spiner",
Memberships = new List<Membership>
{
new Membership { ClubId = club1Id }
}
},
new User
{
Name = "Wesley Crusher",
Memberships = null
}
};
var clubs = new List<Club>
{
new Club
{
Id = club1Id,
Name = "Officers Club"
},
new Club
{
Id = club2Id,
Name = "Captains Club"
}
};
And then I need a LINQ query that adds the matched Club objects to the Membership objects.
So that when I print out users like this:
foreach (var user in users)
{
Console.WriteLine($"User: {user.Name}");
if (user.Memberships != null)
{
foreach (var membership in user.Memberships)
{
Console.WriteLine($" Membership: {membership.Club?.Name}");
}
}
}
It will look like this:
User: Patrick Stewart
Membership: Officers Club
Membership: Captains Club
User: Brent Spiner
Membership: Officers Club
User: Wesley Crusher
I want to do it in one go with a LINQ expression, so that I don't have to involve for-loops.

Answer 1

I ended up solving it with this LINQ expression:
users = users.Select(user => new {
user,
Memberships = user.Memberships?
.GroupJoin(clubs, membership => membership.ClubId, club => club.Id, (membership, club) => new { membership, club})
.SelectMany(mc => mc.club.DefaultIfEmpty(), (mc, club) =>
{
mc.membership.Club = club;
return mc.membership;
})
})
.Select(x =>
{
x.user.Memberships = x.Memberships;
return x.user;
})
.ToList();

Answer 2

LINQ is not intended to be used for modifying data, rather querying data and/or creating new data from data.
Instead, use a nested foreach loop.
First, create a Dictionary to make finding Club objects easier:
var clubDict = clubs.ToDictionary(c => c.Id);
Then, use the clubDict to modify each Membership to include the matching Club entity:
foreach (var user in users.Where(u => u.Memberships != null))
foreach (var membership in user.Memberships)
membership.Club = clubDict[membership.ClubId];