MVC3 DbContext Get New Model's ID After Save?

Question

I have a model Product with a 1:n relationship with model Product_Tag. I also have a model Tag has a 1:n relationship with Product_Tag.

class Product{
  int ID {get;set;}
  public virtual ICollection<Product_Tag> productTag {get;set;}
}

class Tag {
  int ID {get;set;}
  public virtual ICollection<Product_Tag> productTag {get;set;}

}

class Product_Tag {
  int ID {get;set;}
  int ProductID{get;set;}
  int TagID {get;set;}
  public virtual Product product {get;set;}
  public virtual Tag tag {get;set;}
}

When saving a new Product, I want to be able to save any associations to Tags in Product_Tags. Product_Tag requires a PRoductID and a TagID. I have the TagID's available at save time, but the ProductID will come from thew newly-created Product. How to get the newly-created Product's ID immediately after saving the new Product?

Alternatively, is there a way to just save the Product model with its productTags having only TagID's and get DbContext to fill in the ProductID for me?

Answer 1

When you add a new entity to the database, its ID will be automatically populated by Entity Framework. You can use it.

using (var context = new MyDbContext())
{
    var product = new Product();
    product.Name = "My product";

    context.Products.Add(product);
    context.SubmitChanges();

    var productId = product.ID; // here you can get ID of added product
}

Also, EF can work with 'many-to-many' relationships, and you don't need to create additional class for relationship itself. If you use code first, you can use this mapping:

public class Product
{
    int ID {get;set;}
    public virtual ICollection<Tag> Tags {get;set;}
}

public class Tag
{
    int ID {get;set;}
    public virtual ICollection<Product> Products {get;set;}
}

// In your DbContext class:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Product>().HasMany(x => x.Tags).WithMany(x => x.Products)
        .Map(m =>
        {
            m.ToTable("Product_Tag"); // Relationship table name
            m.MapLeftKey("ProductID"); // Name of column for product IDs
            m.MapRightKey("TagID"); // Name of column for tag IDs
        });
}

Then, you can do something like this:

using (var context = new MyDbContext())
    {
        var product = new Product();
        product.Name = "My product";
        product.Tags.Add(new Tag {Name = "furniture"});
        product.Tags.Add(new Tag {Name = "food"});

        context.Products.Add(product);
        context.SubmitChanges();
    }

In this case EF automatically creates a new product, two tags and two relationships between created product and tags.