EF Code First Many to Many relationship creates a duplicate joining table? - mysql

Question

I'm trying to create a many to many relationship between Product and Category with a joining table using EF 6.4.4 and MySQL 8, but unfortunately without success?
public class AppDbContext : DbContext
{
//Schema Tables
public DbSet<Product> Products { get; set; }
public DbSet<Category> Categories { get; set; }
public DbSet<ProductCategory> ProductCategories { get; set; }
}
public abstract class BaseEntity
{
[Key]
public int ID { get; set; }
public string Name { get; set; }
}
public class Product : BaseEntity
{
public virtual IList<Category> Categories { get; set; }
}
public class Category : BaseEntity
{
public virtual IList<Product> Products { get; set; }
}
now for the joining table I tried this:
public class ProductCategory
{
[Key, Column(Order = 1)]
public int ProductID { get; set; }
[Key, Column(Order = 2)]
public int CategoryID { get; set; }
public Product Product { get; set; }
public Category Category { get; set; }
}
and this:
public class ProductCategory : BaseEntity
{
public int ID { get; set; }
[ForeignKey(nameof(Product)), Column(Order = 1)]
public int ProductID { get; set; }
[ForeignKey(nameof(Category)), Column(Order = 2)]
public int CategoryID { get; set; }
public Product Product { get; set; }
public Category Category { get; set; }
}
and also this:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<ProductCategory>().HasKey(x => new { x.ProductID, x.CategoryID });
}
the real problem is no matter what I do EF creates a duplicate joining table?
products
categories
productcategories
productcategory1 (duplicate)
Edit: apparently you can't do this according to this post and this one two, but there is a workaround.
// a workaround for the problem
public abstract class BaseEntity
{
[Key]
public int ID { get; set; }
public string Name { get; set; }
}
public class Product : BaseEntity
{
public virtual IList<ProductCategory> ProductCategories { get; set; }
}
public class Category : BaseEntity
{
public virtual IList<ProductCategory> ProductCategories { get; set; }
}
public class ProductCategory
{
[Key, Column(Order = 1)]
public int ProductID { get; set; }
[Key, Column(Order = 2)]
public int CategoryID { get; set; }
public virtual Product Product { get; set; }
public virtual Category Category { get; set; }
}

Answer 1

I think you have created productcategory class twice. You can remove second one and add fk relationship s in first class definition

Answer 2

You likely need to be explicit with the relationship mapping. I would get rid of the ProductCategories DbSet in the DbContext. If this is EF6 or EF Core 5 then I'd get rid of the ProductCategory entity definition all-together, especially if the table just consists of the two FKs/composite key:
EF 6
modelBuilder.Entity<Product>()
.HasMany(x => x.Categories)
.WithMany(x => x.Products)
.Map(x => {
x.ToTable("ProductCategories");
x.MapLeftKey("ProductId");
x.MapRightKey("CategoryId");
});
EF Core < 5
For EF Core 2/3 the many-to-many needs to be mapped more as a many-to-one-to-many where the collections on each side need to be declared as ProductCategory entities.
public class Product
{
// ...
public virtual ICollection<ProductCategory> ProductCategories { get; set; } = new List<ProductCategory>();
}
public class Category
{
// ...
public virtual ICollection<ProductCategory> ProductCategories { get; set; } = new List<ProductCategory>();
}
EF Core 5
EF Core 5 adds UsingEntity to help define the joining table for many-to-many relationships leaving Product to have Categories (instead of ProductCategories) and Category to have Products (likewise).
modelBuilder.Entity<Product>()
.HasMany(x => x.Categories)
.WithMany(x => x.Products)
.UsingEntity<ProductCategory>(
x => x.HasOne(pc => pc.Product).WithMany().HasForeignKey(pc => pc.ProductId),
x => x.HasOne(pc => pc.Category).WithMany().HasForeignKey(pc => pc.CategoryId));