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ORM- Attaching detached object trees

One of the nice features of an ORM is that it allows you to change a complete object tree and that the ORM will figure out what needs to be added, updated and deleted. The reason why this works is because a typical ORM will track by default all objects that are attached to the ORM abstraction(DBContext in EF Core and ISession in NHibernate). Because it tracks these objects (and their relations) it knows what should happen when you make a change.

Unfortunately when you are sending objects to a client through an API(typically using DTO’s and tools like Automapper) and you get a changed object tree back, the objects in this tree are no longer attached to the ORM, so it doesn’t know what should be inserted, updated or deleted. Luckily most ORM’s are smart and use different tricks to figure out what is changed.

However one place where these ORM’s typically fail is when you are using Automapper and mapping a child collection in your DTO’s to your domain model. An example below where we are FluentNHibernate:

Imagine that we have an existing Order entity with  an OrderDetail collection with following data:

{ Id: 22, OrderId: 49 }

And an OrderDetail DTO arrives with the following data:

{ Id: 22, OrderId: 49 }

{ Id: 23, OrderId: 49 }

This should result in an INSERT operation as 1 new OrderDetail is added. Instead this fails with the following error message:

NHibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: 22, of entity: OrderDetail

The reason why this fails is because Automapper by default creates a new collection when mapping from one type to another. This confuses NHibernate and makes him think that both OrderDetails are new and should be added. (A similar problem exists when using Entity Framework)

A possible fix is to use Automapper.Collection. This will Add/Update/Delete items from a preexisting collection object instead of creating a new collection.

To enable this add the Nuget package to your project and update the Automapper configuration:

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