Composite key entity and dont want to declare PK keys

OK so this should be simple. I have a class

public class ProductConfig
{
   public Category { get;set; }
   public Product { get;set; }
}

These two navigation properties are also primary keys for the table. Declaring PRoductId and CategoryIds are redundat. How can get configure the primary keys using the nav properties?

edit: Stupid me. I forgot something very important in my question above. Those two above are to point out the config. Then we have a third fk thats the selected config for the combination of Product and category. So above entity must be a materialized entity

public class ProductConfig
{
   public Category { get;set; }
   public Product { get;set; }
   public ProductCategoryType { get; set; }
}

---

Declaring ProductId and CategoryId are redundant. How can get configure the primary keys using the nav properties?

Shortly - you can't. While EF6 supports shadow property based FKs, it does not provide a way to configure the PK (and many other column related settings) using the shadow property names - [Key] , [Column] data annotations cannot be applied on navigation property and HasKey fluent API requires primitive property selector expression. In general EF6 does not support shadow properties in PK.

All these limitations have been removed in EF Core. But in EF6, redundant or not, you must define the actual primitive properties in the entity and map them to the composite PK.

---

You have only to set up a relationship between Product and Category entities by navigation properties. EF will set up the correct table structure by its own as many-to-many relationship. So no own relationship entity is needed. Please check this out: many-to-many-relationship in EF

eg:

Product class:

public class Product
{
    // other properties

    public virtual ICollection<Category> Categories { get; set; }
}

Category class:

public class Category
{
    // other properties

    public virtual ICollection<Product> Products { get; set; }
}

Or did I misunderstood your question?

EDIT: IF you need an separate entity like your ProductConfig , than you should try to set it as a unique index constraint by following:

modelBuilder
    .Entity<ProductConfig>()
    .HasIndex(pc => new {pc.Category, pc.Product})
        .IsUnique();

For further information you should read this: HasIndex - Fluent API

EDIT 2 (after getting info solution is for EF < 6.2 needed):

Well after your last question edit, another solution approach is needed. Here we go...

You need a structure like followed:

Product

public class Product
{
    // other properties

    public virtual ICollection<ProductConfig> ProductConfigs { get; set; }
}

Category

public class Category
{
    // other properties

    public virtual ICollection<ProductConfig> ProductConfigs { get; set; }
}

ProductConfig

public class ProductConfig
{
    // other properties

   public virtual Category { get; set; }

   public virtual Product { get; set; }

   public virtual ProductCategoryType { get; set; }
}

To set up a unique constraint in EF < 6.2 you have to do it like that way:

modelBuilder.Entity<ProductConfig>()
        .Property(e => e.Category)
        .HasColumnAnnotation(
            IndexAnnotation.AnnotationName, 
            new IndexAnnotation(new IndexAttribute("YourIndex", 1) { IsUnique = true }));

modelBuilder.Entity<ProductConfig>()
        .Property(e => e.Product)
        .HasColumnAnnotation(
            IndexAnnotation.AnnotationName, 
            new IndexAnnotation(new IndexAttribute("YourIndex", 2) { IsUnique = true }));

modelBuilder.Entity<ProductConfig>()
        .Property(e => e.ProductCategoryType)
        .HasColumnAnnotation(
            IndexAnnotation.AnnotationName, 
            new IndexAnnotation(new IndexAttribute("YourIndex", 3) { IsUnique = true }));

in EF 6.2:

modelBuilder.Entity<Person>()
    .HasIndex(p => new { p.Category, p.Product, p.ProductCategoryType })
    .IsUnique();

EDIT 3 If you have no primary key in your ProductConfig class or you used mine in the example where I added none, because I thought you already have that class. It is possible to set up multiple properties as key. That will result in unique combinations too. You would archive that with the following - instead of the index stuff:

modelBuilder.Entity<ProductConfig>()
            .HasKey(pc => new { pc.Category, pc.Product, pc.ProductCategoryType });

For further information check out the MS docs.

You could also add an Id as primary key, than the indexes are needed.

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