可以使用linq获取值字典吗？

Question

I have solution in my mind, but I am convinced that there must be better solution for the problem.

Lets think I have two tables - User (n..n) Group. In my app I wish to get list of Users, each with all his Groups. In SQL I can achieve similar result with:

SELECT TOP (1000) u.Name, g.Name
FROM [User] u
join [GroupUser] gu on u.Id = gu.UserId
join [Group] g on gu.GroupId = g.Id

In the code I am able to write something like this (please, don't judge the correctness, it's just for illustration):

var result = new Dictionary<Model.User, List<Model.Group>>();
List<string> names = request.Names;
var query = this.dbContext.Set<Model.User>().AsQueryable();

query = from user in query
where names.Contains(user.Name, StringComparer.OrdinalIgnoreCase)
select user;

foreach(var user in await query.ToListAsync())
{
    var groupQuery = 
        from g in this.dbContext.Set<Model.Group>()
        join gu in this.dbContext.Set<Model.GroupUser>() on g.Id equals gu.GroupId
        join u in this.dbContext.Set<Model.User>() on gu.UserId equals u.Id
        where u.Name.Equals(user.Name, StringComparison.OrdinalIgnoreCase)
        select g;

    result.Add(user, await groupQuery.ToListAsync())
}

My question is -- is possible to achieve something like this with single linq query? Or do I really need to enumerate all the Users and fire new query for each of them? This code looks very resource demanding. It is quite simple, only one cycle, but it contains Users.Count+1 query evaluations.

Thanks in advance for hints.

Answer 1

If you have an entity like the following:

public class User
{
    // other properties

    public virtual List<Group> Groups { get; set; }
}

You can query both with an Include :

var users = dbContext.Users.Include(u => u.Groups)
                .Where(u => names.Contains(u.Name, StringComparer.OrdinalIgnoreCase));

Answer 2

Assuming you are using LINQ to SQL, you can combine the queries:

var requestedUsers = from user in query
                     where names.Contains(user.Name, StringComparer.OrdinalIgnoreCase)
                     select user;

var result = (from u in requestedUsers
              join gu in this.dbContext.Set<Model.User>() on u.UserId equals gu.Id
              join g in this.dbContext.Set<Model.Group>() on gu.UserId equals g.Id into gj
              select new { u, gj })
              .ToDictionary(ugj => ugj.u, ugj => ugj.gj.ToList());