How to ignore child collection of circular reference in many-to-many navigation properties

[marc-mueller]

Hello everybody

I am currently struggling configuring AutoMapper for the following scenario

• We have a many-to-many entity mapping between 'Users' and 'Groups'. A group contains a list of users and a user contains a list of groups which the user is member of.
• During an update of a group, we have a list of users of this group in the users collection of this group. The user itself has an empty collection of groups for this use case (focus is updating groups).
• With the above use case, we end up having a group without the user since the user has an empty list of groups which will then remove the many-to-many relationship.

What we need is a mapping definition which would ignore the groups collection in users, if we map the group. if we only map the users, the group list should be mapped as well. So basically we need to ignore the collection depending on which is the main entity we are doing an update in our use case.

On the group mapping definition, we would need something like this:
.ForMember(g => g.Users.Select(u => u.Groups), opt => opt.Ignore())

This does not work and ends up with the following error: Custom configuration for members is only supported for top-level individual members on a type.

Is there a way to configure a mapping for this use case?

Thanks in advance for your help!

Cheers
Marc

[jbogard]

What do your DTOs look like here?

[marc-mueller]

I played a little bit with the API and succeeded in getting the corresponding unit test green. But the current way is a little bit cumbersome and therefore I hope that there is a better way to ignore a member for the use case described above.

The mapping expression which made the unit test go green again is the following:

mappingExpression
    .ForMember(m => m.ParentId, i => i.Ignore())
    .ForMember(m => m.Parent, i => i.Ignore())
    .ForMember(m => m.Users, 
        opt => opt.MapFrom(
            (src, dst, member, ctx) => src.Users?.Select(u =>
            {
                var user = ctx.Mapper.Map<User, User>(u);
                user.Groups = dst.Users?.SingleOrDefault(x => x.Id == user.Id)?.Groups;

                return user;
            })));

Is there a generic way to do that? i.e. setting the ignoremember for Groups in this specific case?

[jbogard]

Probably not? I don't really use AutoMapper for something like this. It's really designed for flattening into a dumb DTO. If you're having to write complex mapping logic, you're far better off just manually writing the code, it's far more maintainable.

[marc-mueller]

Thanks for your feedback. The above code does the trick and I implemented a generic helper for it. I'll need to do more testing if it is always working correctly.

For me it is not a very complex mapping logic since it is a simple circular reference problem with the many-to-many feature of EF Core. On the API level, we cannot assume that the end user always provides the full circular references and therefore normally the DTOs are sent in the context of the corresponding action (i.e. updating the Groups with Users). With ignoring the return-path from users to groups when updating groups, we can build a more stable API in my opinion.

[JoaoEdu93]

Hi @marc-mueller,

In my case, I have a one-to-many relationship and I have a similar problem.
Can you explain to me how you have managed to implement a generic solution, please?

[marc-mueller]

Hi @JoaoEdu93

This discussion is quite old and I cannot remember the exact solution we implemented back then. On the bottom line, we decided to go into another direction. As mentioned, that the mapping logic is not that complicated we implemented a source generator which generates explicit mappers. We have config files (yaml) in our projects were we specify the mappings and the special cases (non default mappings). As Jimmy stated, AutoMapper is not the ideal tool for this use case. We are very happy with our source generator approach. It is simple, straight forward and has good performance.

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