Entity Framework Multiple Column as Primary Key by Fluent Api

Question

These are my simplified domain classes.

public class ProductCategory
{
    public int ProductId { get; set; }
    public int CategoryId { get; set; }

    public virtual Product Product { get; set; }
    public virtual Category Category { get; set; }
}

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

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

This is my mapping class. But it doesn't work.

public class ProductCategoryMap : EntityTypeConfiguration<ProductCategory>
{
    public ProductCategoryMap()
    {
        ToTable("ProductCategory");
        HasKey(pc => pc.ProductId);
        HasKey(pc => pc.CategoryId);
    }
}

How should I map these classes to provide, so that one product can be seen in multiple categories?

Answer 1

Use anonymous type object instead of 2 separated statements:

HasKey(pc => new { pc.ProductId, pc.CategoryId });

From Microsoft Docs: EntityTypeConfiguration.HasKey Method

If the primary key is made up of multiple properties then specify an anonymous type including the properties. For example, in C# t => new { t.Id1, t.Id2 } and in Visual Basic .Net Function(t) New With { t.Id1, t.Id2 } .

Answer 2

Frustratingly, particularly if you happen not to be fluent in lambda, the MSDN VB example is wrong... that should be a 'With', not a 'From'.

(Thanks Aki Siponen )