Code-first database mapping, table missing (wasn't created) for many-to-many relationship - asp.net

Question

I am building ASP.NET webforms application using Entity Framework 6.1, with code-first approach to generate database. I have two tables, Product and Tags, in many-to-many relationship. Classes are below:
public class Product
{
public long Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
}
public class Tag
{
public long Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products{ get; set; }
}
I want two junction tables out of this relationship ProductTags and ProductTagsTradeFor. So I overrided OnModelCreating of WebsiteDbContext.
modelBuilder.Entity<Product>().HasMany<Tag>(s => s.Tags).WithMany(c => c.Products)
.Map(cs =>
{
cs.MapLeftKey("ProductId");
cs.MapRightKey("TagId");
cs.ToTable("ProductTags");
});
modelBuilder.Entity<Product>().HasMany<Tag>(s => s.Tags).WithMany(c => c.Products)
.Map(cs =>
{
cs.MapLeftKey("ProductId");
cs.MapRightKey("TagId");
cs.ToTable("ProductTradeForTags");
});
After running the application, database was created and table ProductTradeForTags is present but table ProductTags was missing.
What is the problem and how do I fix it so both tables are created?

Answer 1

You can't share the navigation properties. You will need to add a second set of navigation collections to each:
public class Product
{
public long Id { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
public virtual ICollection<Tag> TradeForTags { get; set; }
}
public class Tag
{
public long Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Product> Products{ get; set; }
public virtual ICollection<Product> TradeForProducts{ get; set; }
}
Then
modelBuilder.Entity<Product>().HasMany(s => s.Tags).WithMany(c => c.Products)
.Map(cs =>
{
cs.MapLeftKey("ProductId");
cs.MapRightKey("TagId");
cs.ToTable("ProductTags");
});
modelBuilder.Entity<Product>().HasMany(s => s.TradeForTags).WithMany(c => c.TradeForProducts)
.Map(cs =>
{
cs.MapLeftKey("ProductId");
cs.MapRightKey("TagId");
cs.ToTable("ProductTradeForTags");
});

Answer 2

Your model require to navigate from a Tag to Products and from a Product to Tags.
In this case one association table is enough.
EF should raise an exception but it simply ignores the first configuration.