EF CF - 设置外键不设置实体

Question

我有一个Client和一个Assignmnet类。关系是客户可以有多个任务。

public class Assignment
{
    public DateTime created { get; set; }
    public DateTime? updated { get; set; }

    [ForeignKey("clientId")]
    public virtual Client client { get; set; }
    [ForeignKey("client")]
    public int clientId { get; set; }        

    [Key]
    public int id { get; set; }   
}

public class Client
{
    public DateTime created { get; set; }
    public DateTime? updated { get; set; }

    public virtual IEnumerable<Assignment> assignments { get; set; }

    [Key]
    public int id { get; set; }
}

以下代码有效。客户端和clientId都设置为正确的值。

var clientController = GetClientController();
Client c = new Client();
clientController.PostClient(c);

var assController = GetAssignmentController();
Assignment a = new Assignment();
a.client = c;
assController.PostAssignment(a);

但以下代码不起作用。 clientId已更新，但客户端为空。

var clientController = GetClientController();
Client c = new Client();
clientController.PostClient(c);

var assController = GetAssignmentController();
Assignment a = new Assignment();
a.clientId = c.id;
assController.PostAssignment(a);

分配控制器的工作原理如下：

public HttpResponseMessage PostAssignment(Assignment assignment)
{
    if (!ModelState.IsValid)
    {
        return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
    }

    try
    {
        _db.Insert(assignment);
        _db.Save();
        HttpResponseMessage response = Request.CreateResponse(HttpStatusCode.Created, assignment);
        return response;
    }
    catch (Exception ex)
    {
        return Request.CreateErrorResponse(HttpStatusCode.NotFound, ex);
    }
}

有人知道我做错了吗？