Question

How does one map many to many relationships?

One-to-one relationships are easy….

Assuming…

public class ProductDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int SupplierId { get; set; }
}

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Cost { get; set; }
    public Supplier Supplier { get; set; }
}

public class Supplier
  {
    public int Id { get; set; }
    public string Name { get; set; }
    public int Rating { get; set; }
    public ICollection<Product> Products { get; set; }
}

 Mapper.CreateMap<Product, ProductDTO>()
            .ForMember(d => d.SupplierId, m => m.MapFrom(s => s.Supplier.Id));

Works assuming every product has only one supplier and a supplier can have many products. How do I map it if a product can also have many suppliers?

I changed supplier line in Products to

 public ICollection<Supplier> Supplier { get; set; }

and in ProductDTO doing the same

 public ICollection<int> SupplierId { get; set; }

How do I alter CreateMap since the Ids are now collections? Autocomplete no longer shows Id and all I get are functions.

I’m new to C# so I many be missing something obvious. Am I supposed to iterate in a for loop and map the ids one by one?

Answer 1

You may try to use:

Mapper.CreateMap<Product, ProductDTO>()         
      .ForMember(d => d.SupplierIds, m => m.MapFrom(p => p.Suppliers.Select(s => s.Id)));

Another option:

Mapper.CreateMap<Supplier, int>().ConvertUsing(s => s.Id);
Mapper.CreateMap<Product, ProductDTO>()                                
   .ForMember(d => d.SupplierIds, m => m.MapFrom(p => p.Suppliers));

One more thing if you are this using DTO to pass data from Web/WCF service you may consider using

public ICollection<SupplierDTO> Supplier { get; set; }

instead if passing supplier ids only. In most cases it’s better (and more effective) to pass more data in one call to the service than doing few calls.