How to use AutoMapper to assign reference based on key value?

My use case is simple. I have a root domain object which references a child object. I have a DTO passed back from a service call that represents the domain object but rather than pass the child object each time, the DTO contains a property that holds the child object's key value. Something like this:

public class DomainObject
{
    public ChildObject Child { get; set; }
    public String Name { get; set; }
}

public class ChildObject
{
    public Int32 Key { get; set; }
    public String Name { get; set; }
}

public class DTO
{
    public Int32 ChildKey { get; set; }
    public String Name { get; set; }    
}

I have a cached list开发者_如何转开发 of ChildObjects. When I map from DTO=>DomainObject I want to set the DomainObject.Child property to the existing instance of ChildObject from the cache using the DTO.ChildKey property. Does this require a custom value resolver or is there another way to accomplish this?

---

Yes you will need a custom value resolver. Something like this will do it:

public class KeyToChildObjectResolver : ValueResolver<Int32, ChildObject>
{
    protected override ChildObject ResolveCore(Int32 source)
    {
         return Cache.Get<ChildObject>(source);
    }
}

And then:

Mapper.CreateMap<DTO, DomainObject>()
      .ForMember(x => x.Child, o => o.ResolveUsing<KeyToChildObjectResolver>()
                                     .FromMember(x => x.ChildKey));

You could do it with a Resolver that goes straight from DTO to ChildObject but then your resolver is essentially single purpose. This way you can use it anywhere you have a child key to be mapped to a ChildObject