Question

Is it possible to define a many-to-many relationship in Entity Framework 4.1 (Code First approach) using Data Annotations only, without model builder?

For example, something like:

Product = { Id, Name, ... }
Category = { Id, Name, ... }
ProductCategory = { ProductId, CategoryId }

You get the picture.

I don’t want to have an intermediate entity ProductCategory in the context with two many-to-ones since I don’t have any additional data, just the two FKs. Also, I should be able to define table name for the intermediate table for use with an existing database.

Answer 1

It is possible to define many-to-many with default conventions or with data annotations but it is not possible to change mapping to junction table (table’s name and columns) without model builder. Simple many-to-many:

public class Product
{
    public int Id { get; set; }
    public virtual ICollection<Category> Categories { get; set; }
}

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

For using annotations you can use:

public class Product
{
    [Key]
    public int Id { get; set; }
    [InverseProperty("Products")]
    public virtual ICollection<Category> Categories { get; set; }
}

public class Category
{
    [Key] 
    public int Id { get; set; }
    [InverseProperty("Categories")]
    public virtual ICollection<Product> Products { get; set; }
}

If you need to control mapping of junction table to existing database you need modelBuilder. Data annotations are not as powerful as fluent API.