如何在Entity Framework模型中正确定义一对多关系？

Question

I have two entities in database: customer and user and building a one-to-many relationship between them, so 1 customer -> many users.

This is my customer model:

public class Customer
{           
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

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

What would be the correct User model, I mean with Customer property and CustomerId property or just CustomerId? So this:

public class User
{       
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    public Customer Customer { get; set; }

    [ForeignKey("Customer")]
    public int CustomerId { get; set; }
}

.. or this?:

public class User
{       
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [ForeignKey("Customer")]
    public int CustomerId { get; set; }
}

Answer 1

Actually it should be like the code example below. You should specify what is local primary key, remote key and if there is navigation property ( Customer ), then You should specify "How should it recognize the object" => based on the this or that key of Customer ( CustomerId ).

public class User
{       
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    //other properties

    [ForeignKey("CustomerId")]
    public Customer Customer { get; set; }

    public int CustomerId { get; set; }
}

If You are not interested in having navigation property, You can simply avoid that to have only:

public class User
{       
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    //other properties

    public int CustomerId { get; set; }
}

More info can be found there: http://www.entityframeworktutorial.net/code-first/foreignkey-dataannotations-attribute-in-code-first.aspx

---

Note: You do not need to specify [ForeignKey] attribute, if You are using Navigation Property with the same name (eg Customer & CustomerId ).

If there are multiple references to the same class (eg You have User that will have exactly 2 Customers (CustomerA, CustomerB), then You must specify following:

public class User
{       
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    //other properties


    [ForeignKey("CustomerIdA")]
    public Customer CustomerA { get; set; }
    public int CustomerIdA { get; set; }

    [ForeignKey("CustomerIdB")]
    public Customer CustomerB { get; set; }
    public int CustomerIdB { get; set; }
}

Answer 2

In the first case you have a correctly set navigational property between User and Customer. In the second case, you cannot navigate from User to Customer, and you have a Foreign Key attribute that point to a non-existent property.

You should, however, invert the order of the ForeingKeyAttribute to make it clearer (both ways are OK, though):

[ForeignKey("CustomerId")]
public Customer Customer { get; set; }

public int CustomerId { get; set; }

If you won't need User.Customer , you can skip setting up the navigational property altogether. The same applies to Customer.Users , you can skip setting up that navigational property if it's not needed.