在添加新的导航属性后，实体框架尝试在查询时将属性设置为null

Question

I add a navigation property from Customer to Purchase (as a collection) in my Entity Framework project. The relationship already existed before, so no database migration is performed.

When I retrieve a non-empty set of Product - which also has a navigation property to Purchase (as a collection) - from the context, I get the following exception:

The 'BrandID' property on 'BrandedProduct' could not be set to a 'null' value. You must set this property to a non-null value of type 'System.Int32'.

Simplified code (all entities also have an ID property and a protected/public empty constructor):

public class Customer {
    // Adding this line causes the exception. Without it, everything works fine.
    public virtual ICollection<Purchase> Purchases { get; private set; }
}

public abstract class Product {
    public virtual ICollection<Purchase> Purchases { get; private set; }
}

public class BrandedProduct : Product {
    // This is the property that EF tries to set to null
    public int BrandID { get; private set; }

    public virtual Brand Brand { get; private set; }
}

public class Brand {}

// This works as a many-to-many relationship between Product and Customer
public class Purchase {
    public int CustomerID { get; private set; }
    public int ProductID { get; private set; }

    public virtual Customer Customer { get; private set; }
    public virtual Product Product { get; private set; }

    // Other properties...
}

The Purchase class has a composite primary key, so the following configuration is set up in the DataContext class.

public class DataContext : DbContext {
    protected override void OnModelCreating(DbModelBuilder builder) {
        builder.Entity<Purchase>().HasKey(x => new { x.CustomerID, x.ProductID });
    }
}

Any idea why this could happen?

Answer 1

In this example, it looks like the problem is due to BrandedProduct not having a primary key which is required by Entity Framework. It is attempting to figure out and set BrandID as the primary key, which has a private setter.

To solve this, just add the [Key] attribute or another HasKey to your configuration for a property with a public setter.