DDD and references between aggregates in EFCore and C#

c# domain-driven-design entity-framework-core


I have an issue that I am not sure how to solve when DDD is assumed and using C#/EF Core.

Simplified situation: We have 2 aggregates - Item and Warehouse. Each of them has its identity by ExternalId(Guid) to identify it outside (FE etc) which is also treated as its domain identity. It also has database Id that identifies it inside databases model - Entity model and Db model is the same class as EF Core allows to use private fields - only the ExternalId, and required fields are exposed. Entity (both in DDD and EF Core sense) contain quite a lot of business logic and methods strictly coupled to the object. In general I follow the pattern from eShop/eShopOnContainers example.

Item is assigned to Warehouse and when creating an item we need to pass Warehouse to its constructor.

Is it proper to pass full Warehouse object to Item's constructor (but also to other methods that Item defines):

public Item(Warehouse warehouse,..)

or should I relay on database Id only :

public Item(long warehouseId,..)

I have an issue about this, because from one one side I read that aggregates should not reference other aggregates, but on the other hand using Datbase DB leaks the implementation detail (object persitsance in relational DB) to domain model which should not happen in my opinion.

Using ExternalId:

public Item(Guid warehouseId,..)

does not solve the problem as actual relations in db do not base on it.

What is your opinion ? I am a bit puzzled.

9/25/2019 12:14:59 PM

Popular Answer

Usually you would create a Value Object for the Id of the Aggregate Root. It is one possibility to rely on a Id generated by the database. If you decide to let the Db generate the Id, then you will need to work with that. But why would you need to pass the Warehouse reference or Id anyways? It looks like Item is an Entity and Warehouse is the Aggregate Root that should contain that Entity. In general you should not create an Entity outside of the Aggregate Root.

Edit: There are several identity creation strategies as Vaughn Vernon describes in the red book. One of them is let the persistance mechanism such as a SQL Db generate the unique identifier of an entity or aggregate.

3/3/2019 6:41:43 AM

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