You can rename the Category properties in your ProductOrders class to something like MainCategory and SecondaryCategory by using data annotations. Specifically, you can use the [Column] attribute to specify the column name for each property.

Here’s how you can do it:

  using System.ComponentModel.DataAnnotations.Schema;
    
    namespace API.Entities
    {
        [Table("ProductOrders", Schema = "product_orders")]
        public class ProductOrders
        {
            public int Id { get; set; }
    
            // Rename CategoryId to MainCategoryId
            [Column("CategoryId")]
            public int? MainCategoryId { get; set; }
    
            // Rename Category to MainCategory
            [ForeignKey("MainCategoryId")]
            public Category? MainCategory { get; set; }
    
            // Add a secondary category with a new column name
            [Column("SecondaryCategoryId")]
            public int? SecondaryCategoryId { get; set; }
    
            [ForeignKey("SecondaryCategoryId")]
            public Category? SecondaryCategory { get; set; }
        }
    }

Renaming CategoryId and Category: refer to the tutorials: Column names, ForeignKeyAttribute

For example:

[Table("ProductOrders", Schema = "product_orders")]
    public class ProductOrders
    {
        public int Id { get; set; }

        [Column("CategoryId")]
        public int? MainCategoryId { get; set; }
        
        [ForeignKey("MainCategoryId")]
        public Category? MainCategory { get; set; }
    }

Edit:

About the second question:

You cannot do that.

The exception ”ProductOrders.MainCategoryId’ and ‘ProductOrders.SecondCategoryId’ are both mapped to column ‘CategoryId’ in ‘ProductOrders’, but the properties are contained within the same hierarchy. All properties on an entity type must be mapped to unique different columns.’ will be thrown while attempting to create an instance.

So, you can achieve this using EF Core’s foreign key configuration.

For example:

public class Category
{
    public int CategoryId {  get; set; }
    [Required]
    public string Name { get; set; } = string.Empty;
}

public class ProductOrders
{
    public int Id { get; set; }

    public int? MainCategoryId { get; set; }

    [ForeignKey("MainCategoryId")]
    public Category? MainCategory { get; set; }

    public int? SecondCategoryId { get; set; }

    [ForeignKey("SecondCategoryId")]
    public Category? SecondCategory { get; set; }
}

In DbContext:

The HasForeignKey(p => p.SecondCategoryId) method specifies that SecondCategoryId will serve as a foreign key referencing the Category entity. SecondCategoryId is the foreign key column in the ProductOrders table that points to the Category table.

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options)
    : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // configure the foreign key 'MainCategoryId'
        modelBuilder.Entity<ProductOrders>()
            .HasOne(p => p.MainCategory)
            .WithMany()
            .HasForeignKey(p => p.MainCategoryId)
            .OnDelete(DeleteBehavior.Restrict);

        // configure the foreign key 'SecondCategoryId'
        modelBuilder.Entity<ProductOrders>()
            .HasOne(p => p.SecondCategory)
            .WithMany()
            .HasForeignKey(p => p.SecondCategoryId)
            .OnDelete(DeleteBehavior.Restrict); 

        base.OnModelCreating(modelBuilder);
    }

    public DbSet<Category> Category { get; set; }
    public DbSet<ProductOrders> ProductOrders { get; set; }
}