Entity Framework Core relationship tracking

Question

I have a 0-to-many relationship between Product and Category, configured as follows:

public class Product 
{

    public int Id { get; set; }
    public string Name { get; set; }
   
    public int? CategoryId { get; set; }
    public Category Category { get;set; }
}

public class Category
{
     public int Id { get; set; }
     public string Name { get; set; }
}

When I try to manipulate data (and save), I have a strange behaviour with the CategoryId

// ...

var cat1 = this.Context.Categories.Find(1);
var cat2 = this.Context.Categories.Find(2);

var product1 = new Product();
product1.Name = "Test";
product1.Category = cat1;  // the CategoryId property is NOT set

this.Context.Products.Add(product1);  // the CategoryId property is set
this.Context.SaveChanges();

product1.Category = cat2;  // the CategoryId property is NOT updated

this.Context.SaveChanges();  // the CategoryId property is updated

Is this behaviour correct? Because I would have expected that, once in tracker, the CategoryId field to be updated when the Category field is updated...

Am I wrong or am I doing something wrong? I cannot find anything in the docs about this...

Thanks in advance

Answer 1

This is the correct behavior. Entity is updated only after SaveChanges.

But if you need to update CategoryId immediately

product1.CategoryId = cat2.Id;

and it is a better way to update. Sometimes you will need to add after this, before SaveChanges:

Context.Entry(product1).State = EntityState.Modified;

And by the way, to get 0-to-many you have to fix your Category class

public class Category
{
     public int Id { get; set; }
     public string Name { get; set; }
      public virtual ICollection<Product> Products { get; set; }
}