在父对象内映射子对象

Question

I'm having a bit of an issue. I don't quite know how to handle the situation so I'll just explain a simplified scenario and hopefully you can help me.

I'm trying to map a parent database object to a parent bll object. In this parent database object, there is a foreign key to the ID of the child, and in my parent bll object I use the child bll object (containing more than just an ID).

So here are my bll objects:

public class Parent
{
    public int ID { get; set; }
    public Child Child { get; set; }
}

public class Child
{
    public int ID { get; set; }
    public string FirstName { get; set; }
}

And here is my mapper class/method:

public class ParentMapper
{
    public Parent MapFromSource(ParentDatabaseObject parentDO)
    {
        Parent parent = new Parent();
        parent.ID = parentDO.ID;
        parent.Child = ???;
        return parent;
    }
}

I don't think it's very important what the ParentDatabaseObject looks like in this case, I'd just like to know how I should map the parent.Child object.

I have considered the following:

parent.Child = new Child();
parent.Child.ID = doParent.Child.Id;
parent.Child.FirstName = doParent.Child.FirstName;

Which doesn't feel right, 'cause I kind of have the urge to put this in my ChildMapper, which leads me to my second way of implementing this (assuming I have a seperate child mapper and have an instance of it called childMapper):

parent.Child = childMapper.MapFromSource(parentDO.Child);

But I kind of have the feeling that using this way of mapping is making my code a bit tightly coupled, because I'd be using my ChildMapper in my ParentMapper.

So I guess my question is: how should I implement this kind of mapping. Is this last method correct or is there something even better? I'm already discarding the first thing I tried.

Thanks for your help!

(I did research before posting this question and this was the closest I could find: Data Mapper for Child Objects , but I wasn't really content with the only answer in there)

Answer 1

Shouldn't it be better -

parent.Child = childMapper.MapFromSource(parentDO.FoeignKeyToTheChild);  

I think you should have methods to get object by Id.

EDIT : If your mapper doesn't DataAccess code, then you have to map the child within your Repository . As your Repository already have DataObjects ready, you can do it the following way -

ParentMapper:

public class ParentMapper
{
    public Parent MapFromSource(ParentDo parentDo)
    {
        Parent parent = new Parent();
        parent.Id = parentDo.Id;
        return parent;
    }
}  

ChildMapper:

public class ChildMapper
{
    public Child MapFromSource(ChildDo childDo)
    {
        Child child = new Child();
        child.Id = childDo.Id;
        child.FirstName = childDo.FirstName;

        return child;
    }
}  

Repository:

public class Repository
{
    //you already have parentDo
    //you already have childDo

    public Parent GetParent()
    {
        Parent parent = parentMapper.MapFromSource(parentDo);
        parent.Child = childMapper.MapFromSource(childDo);
        return parent;
    }

    public Child GetChild()
    {
        Child child = childMapper.MapFromSource(childDo);
        return child;
    }
}  

Otherwise, your Mapper must have access to DataAccess code.