如何使用实体框架查询多对多实体

Question

1. User has many Roles
2. Task has many Receivers ( type of Role )

User -> Role

Task -> Role

How can I write a query to get User tasks? I tried code below, but it returns an IEnumerable<ICollection> ;

v var userTasks = from role in Context.Users.Find(userId).Roles
            join taskRole in Context.Tasks.SelectMany(t => t.Receivers) on role.Id equals taskRole.Id
            select taskRole.Tasks;

Answer 1

In your SelectMany, store the corresponding task

Context.Users.Find(userId).Roles
.Join(Context.Tasks.SelectMany(t => t.Receivers.Select(tr => new {t, tr})), 
    r => r.Id, 
    t => t.tr.Id, 
    (r, t) => t.t)
.Distinct() // if task override Equals/HashCode

Answer 2

Use the double from clause instead of the join:

var userTasks = from role in Context.Users.Find(userId).Roles
                from task in role.Tasks
                select task;

Or if you prefer the short way:

var userTasks = Context.UserAccount.Find(userID).Roles.SelectMany(role => role.Tasks);