Re: [DDD/CQRS] Checks before deleting an entity

[Boris Guéry]

As far as a business rule is involved (invariant), your read-model (which is eventually consistent) must not be used.

Maybe you need to distill your design, and make sure your invariants cannot be broken.

A Manufacturer cannot be disabled if it has any series linked to it.

Note: I said disabled because I wanted to point you are going to actually do a soft delete. Your aggregate will probably still exists.

However, there is nothing wrong to check if there are series linked to your manufacturer from your read-command before to issue your commands, but you should not trust this information.

I guess your Manufacturer must be aware of the Series linked to it and not the opposite.

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Boris Guéry - IT Consultant / Software Architect

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On Fri, Jun 2, 2017 at 11:54 AM, 'Newm' via DDD/CQRS <ddd...@googlegroups.com> wrote:

I have started experimenting with CQRS/ES and have created a very basic project. The idea is that I need to record details of model cars such as Matchbox, Corgi, Dinky, etc.

I have broken it down into 3 aggregate roots:

Manufacturer (Matchbox, Corgi, Dinky)

Series (Matchbox->Hot Wheels, Matchbox->75s, etc)

Vehicles

A series must have a manufacturer, a vehicle must have a series.

I have created a number of commands and events to allow the creation of manufacturers and series e.g.

CreateManufacturer

CreateSeries

CreateVehicle

Before allowing the user to delete a manufacturer I would like to check that there are no series linked to it.  Similarly I would like to ensure that there are no vehicles linked to a series before allowing it to be deleted.

How should this be handled?  Should I create a service which uses the read models to determine if a manufacturer/series is in use before sending a DeleteManufactuer/DeleteSeries command or should the manufacturer/series domain objects (aggregate roots) internally track the number of items linked to it using the Apply override (i'm using CQRSLite)?

If I go down the domain object path should the CreateSeries command be replaced with a AddSeries method on the manufacturer domain object?

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