Duplicate instances despite EqualityComparison

[FBryant87]

Tried to keep this simple so hopefully it's clear:

DB Forest: I have a Forest which has a Tree in it. There are Sections in my Forest, but these Sections currently have no Trees in them.

Changed Forest: I simply add the Tree object to one of the Sections (there is only one Tree instance still)

This illustrates the small change: https://imgur.com/a/hahD7

I load the DB Forest from the database, and map the Changed Forest to the DB Forest.

I want Entity Framework to automatically detect that a tree has been added to the section, which it does, but it detects it as a newly created tree instance (rather than use the existing object). Thus resulting in duplicate trees.

When I map the objects, I try to tell AutoMapper.Collection to detect this equivalency and not create any new objects, but use the existing ones:

var config = new MapperConfiguration(cfg => {
                cfg.AddCollectionMappers();
                cfg.CreateMap<Forest, Forest>().PreserveReferences().EqualityComparison((s, d) => s.ID == d.ID);
                cfg.CreateMap<Tree, Tree>().PreserveReferences().EqualityComparison((s, d) => s.ID == d.ID);
                cfg.CreateMap<Section, Section>().PreserveReferences().EqualityComparison((s, d) => s.ID == d.ID);
            });
.

The IDs of both trees are identical, but AutoMapper still creates 2 separate tree objects. How might I prevent this? Many thanks.

(EF 6.1.3, AutoMapper 6.0.2, AutoMapper.Collection 3.1.2)

[TylerCarlson1]

So what I'm seeing is Tree is tied to the Forest and a Section, so there's 2 parent child relationships to Tree. If that's the case AM.Collections can't help because it has no idea about the dual parent relationships. The only way to do this is do an after mapping which corrects the issue, or don't map to sections and pass the matching Forest.Trees object in the DB to the sections, so it doesn't generate a new object.

Also are you using a DTO for your classes? I see it looks like you are cloning here, which might be part of the problem.

[FBryant87]

Ah so multiple parent relationships are the problem here, many thanks - will close.