composite key as foreign key
C#Entity FrameworkEf Code-FirstForeign KeysComposite KeyC# Problem Overview
I am using Entity framework 4.1 in MVC 3 application. I have an entity where I have primary key consists of two columns ( composite key). And this is being used in another entity as foreign key. How to create the relationship ? In normal scnerios we use :
public class Category
{
public string CategoryId { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
public class Product
{
public int ProductId { get; set; }
public string Name { get; set; }
public string CategoryId { get; set; }
public virtual Category Category { get; set; }
}
but what if category has two columns key ?
C# Solutions
Solution 1 - C#
You can use either fluent API:
public class Category
{
public int CategoryId1 { get; set; }
public int CategoryId2 { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
public class Product
{
public int ProductId { get; set; }
public string Name { get; set; }
public int CategoryId1 { get; set; }
public int CategoryId2 { get; set; }
public virtual Category Category { get; set; }
}
public class Context : DbContext
{
public DbSet<Category> Categories { get; set; }
public DbSet<Product> Products { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<Category>()
.HasKey(c => new {c.CategoryId1, c.CategoryId2});
modelBuilder.Entity<Product>()
.HasRequired(p => p.Category)
.WithMany(c => c.Products)
.HasForeignKey(p => new {p.CategoryId1, p.CategoryId2});
}
}
Or data annotations:
public class Category
{
[Key, Column(Order = 0)]
public int CategoryId2 { get; set; }
[Key, Column(Order = 1)]
public int CategoryId3 { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
public class Product
{
[Key]
public int ProductId { get; set; }
public string Name { get; set; }
[ForeignKey("Category"), Column(Order = 0)]
public int CategoryId2 { get; set; }
[ForeignKey("Category"), Column(Order = 1)]
public int CategoryId3 { get; set; }
public virtual Category Category { get; set; }
}
Solution 2 - C#
I believe the easiest way is to use Data Annotation on the Navigation property like this:
[ForeignKey("CategoryId1, CategoryId2")]
public class Category
{
[Key, Column(Order = 0)]
public int CategoryId1 { get; set; }
[Key, Column(Order = 1)]
public int CategoryId2 { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products { get; set; }
}
public class Product
{
[Key]
public int ProductId { get; set; }
public string Name { get; set; }
public int CategoryId1 { get; set; }
public int CategoryId2 { get; set; }
[ForeignKey("CategoryId1, CategoryId2")]
public virtual Category Category { get; set; }
}
Solution 3 - C#
In .NET Core
and .NET 5 <
the documentation only shows Data annotations (simple key)
.
However using the example from @LadislavMrnka you will get a error message like this:
> System.InvalidOperationException: There are multiple properties with > the [ForeignKey] attribute pointing to navigation ''. To define a > composite foreign key using data annotations, use the [ForeignKey] > attribute on the navigation.
Using that error message you can write the code like this:
public class Product
{
[Key]
public int ProductId { get; set; }
public string Name { get; set; }
public int CategoryId2 { get; set; }
public int CategoryId3 { get; set; }
[ForeignKey("CategoryId2,CategoryId3")]
public virtual Category Category { get; set; }
}
Fluent API (composite key)
example from Microsoft:
internal class MyContext : DbContext
{
public DbSet<Car> Cars { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Car>()
.HasKey(c => new { c.State, c.LicensePlate });
modelBuilder.Entity<RecordOfSale>()
.HasOne(s => s.Car)
.WithMany(c => c.SaleHistory)
.HasForeignKey(s => new { s.CarState, s.CarLicensePlate });
}
}
public class Car
{
public string State { get; set; }
public string LicensePlate { get; set; }
public string Make { get; set; }
public string Model { get; set; }
public List<RecordOfSale> SaleHistory { get; set; }
}
public class RecordOfSale
{
public int RecordOfSaleId { get; set; }
public DateTime DateSold { get; set; }
public decimal Price { get; set; }
public string CarState { get; set; }
public string CarLicensePlate { get; set; }
public Car Car { get; set; }
}