如何使用实体框架在C＃中的多对多关系上运行linq查询

Question

I am having problems querying many-to-many relationships in Linq To Entities. I am basically trying to replicate this query using Linq:

SELECT  
    Users.UserName, Roles.RoleName, Users.IsActive 
FROM 
    Users
LEFT JOIN 
    UserRoles ON Users.UserID = UserRoles.UserID
LEFT JOIN 
    Roles ON UserRoles.RoleID = Roles.RoleID

I tried this query in Linqpad and it works absolutely fine

Users.Join(UserRoles, u=>u.UserId, r=>r.UserId, (u,r) => new { u, r})
      .Join(Roles, ur=>ur.r.RoleId, q=>q.RoleId, (ur,q)=>new {ur, q})
      .Select(m=> new {m.ur.u.UserName, m.ur.u.IsActive, m.q.RoleName})

But when I am trying the same query in my C# application it throws an error

Error

'Context' does not contain a definition for 'UserRoles' and no extension method 'UserRoles' accepting a first argument of type 'Context' could be found (are you missing a using directive or an assembly reference?)

My question is how to create a LINQ query in C# to retrieve Users which are assigned to Roles ?

My DbContext contains

public class Context : DbContext
{
    public Context()
        : base("Name=xyz")
    {
        if (!Database.Exists("xyz"))
            Database.SetInitializer(new Initializer());
    }
    public DbSet<User> Users { get; set; }
    public DbSet<Role> Roles { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new UserMap());

        base.OnModelCreating(modelBuilder);
    }
}

public class UserMap : EntityTypeConfiguration<User>
{
    public UserMap()
    {
        HasMany(r => r.Roles).WithMany(u=>u.Users)
            .Map(m =>
            {
                m.ToTable("UserRoles");
                m.MapLeftKey("UserId");
                m.MapRightKey("RoleId");
            });
    }
}

public class User
{
    public User()
    {
        this.Roles = new HashSet<Role>();
    }
    public int UserId { get; set; }
    public string UserName { get; set; }
    public string PasswordHash { get; set; }
    public DateTime? LastLoggedOn { get; set; }
    public DateTime? PasswordChangedOn { get; set; }
    public int IsActive { get; set; }

    public virtual ICollection<Role> Roles { get; set; }
}

public class Role
{
    public Role()
    {
        this.Users = new HashSet<User>();
    }
    public int RoleId { get; set; }
    public string RoleName { get; set; }
    public string Description { get; set; }

    public virtual ICollection<User> Users { get; set; }
}

Answer 1

EDIT: Looking at the code in the question: there is no UserRoles defined. You need to refactor in terms of EF knowing the many-many relationship:

var res = db.Users.SelectMany(u => 
                  u.Roles.Select(r =>
                      new {
                        u.UserName,
                        r.RoleName,
                        u.IsActive
                      }
                  )
          );

EF understands (because you've defined it) how to do the join: The Roles collection – representing the relationship – will filter on only the roles belonging to the user that you start with.

---

Original Answer

In what type is the definition of the method in which you write the query?

LINQpad (IIRC) runs everything as if it were a method of your DbContext derived type. Thus the DbSet<T> properties mapping tables are usable without qualification (in your case UserRoles ). If not a member of your context type you need to qualify the entity sets with an instance of the context:

using (var db = new MyContext()) {
  var data = db.Users.Join(db.UserRoles, u=>u.UserId, r=>r.UserId, (u,r) => new { u, r})
               .Join(db.Roles, ur=>ur.r.RoleId, q=>q.RoleId, (ur,q)=>new {ur, q})
               .Select(m=> new {m.ur.u.UserName, m.ur.u.IsActive, m.q.RoleName})
}