使用 Entity Framework 6 将父实体与数据库中现有的子实体连接起来

Question

I'm using Entity Framework 6 to design and use a database that has a child Table of users which looks like this:

public class User
{
    public long Id { get; set; }
    public string UserName { get; set; }
    public string CompanyId { get; set; }
}

And an Order table that looks like this:

    public class Order
{
    public long OrderId { get; set; }
    public int ItemCount { get; set; }
    public DateTime OrderDate{ get; set; }
    ...

    public virtual User User { get; set; }

}

In my database, the field representing the User table in Order table is User_Id

Now my issue here (which might be a database design issue) is that before Creating a new User and Order in the database, I want to check if the user exists, and if he does, link it to the new order instead of creating a new User. I have a method that Adds new user if the user doesn't exist and returns either the created or the existing user's Id field value.

How to Create a new instance of Order that takes the Id value instead of User object to avoid having duplicate users, or is there a better way to handle this Use Case?

I tried to return the object instead of the UserId, but it saves a new User in the User Table

Answer 1

Not sure if this answers your problem. But this way should ensure that you don't create another user object, but instead retrieves a user object from db.

 public class UserHandler
    {
        //Use dbcontext instead to get a user list
        public List<User> UserList = new List<User>();
        public void handleUserList(int id)
        {
            User oneUser = UserList.FirstOrDefault(n => n.id==id);
            if (oneUser.Equals(null))
            {
                oneUser = new User();
                //Functionality to save the user information
            }
        }
    }

Another tip is to use data annotations for properties to make sure they will act the way you want.

public class Order
{
    [Key]
    public long OrderId { get; set; }
    public int ItemCount { get; set; }
    public DateTime OrderDate{ get; set; }
    ...

    public virtual User User { get; set; }

}