Comments on: Entities, Value Objects, Aggregates and Roots

By: Taking executable specs to the next level: Executable Documentation « KeithPS

Sun, 26 Jun 2011 15:57:07 +0000

[...] morning I had a look at the specs for one of our aggregate roots that resulted from its TDD to see if it was adequately documenting its public API, lifecycle [...]

By: Cascade Save-Update in NHibernate and version column « FrankMao.com

Cascade Save-Update in NHibernate and version column « FrankMao.com — Mon, 02 May 2011 14:42:55 +0000

[...] complies with aggregate root pattern,as one of the aggregate rules is: When a change to any object within the Aggregate boundary is committed, all invariants of the [...]

By: Chriz

Chriz — Fri, 12 Jun 2009 16:43:49 +0000

@bogardj
Normaly when working with orders, you have something like an order number. And in many cases you will access you order-lines only by an order. But imagine we need a list of orderlines of a special product. Something like “give me all orderlines of products from toshiba” to show them on the screen. Imagine each order could hold hundrets of orderlines. Accessing them through their orders would be impossible.

By: bogardj

bogardj — Tue, 09 Jun 2009 17:41:58 +0000

@Chriz

How is the information viewed? Who is viewing it? These questions lead me to what my boundaries are, not only for aggregates, but services and systems.

By: Chriz

Chriz — Mon, 08 Jun 2009 20:17:06 +0000

I have some problem with the “Only Aggregate Roots can be obtained directly with database queries”-Rule. Take the following Use-Case as an example. I have Orders and Order-Lines. Orders are the rootEntity of OrderLines. OrderLines have a product-type like “Toshiba Notebook N750″. Now I need to do some work with OrderLines of a special product-type. Therefore I need to pull them from the repository. If I would use the Order-Repository it would pull me out all Orders with at least one OrderLine of the desired type and of course all the other OrderLines of those orders. That won’t be acceptable from a performance point of view. How can I circumvent this ? Only lazyload the other OrderLines ?

By: kengkaj

kengkaj — Fri, 26 Sep 2008 08:04:36 +0000

Thanks for your great article.

By: Tudor-Andrei Pamula

Tudor-Andrei Pamula — Fri, 12 Sep 2008 14:33:10 +0000

Besides the above link MocksArentStubs I could suggest http://googletesting.blogspot.com/2008/07/breaking-law-of-demeter-is-like-looking.html.

The point that if you have code depending on a navigation a.B.C.D then in the unit-test you’ll have to create/stub mock A & B & C & D. Either you go with slow integration tests with real domain-objects, or you go with object-mothers and (in both cases you’ll have to create real objects, probably through factories which might contain aspects/entities irrelevant to your test). Or you try to create mocks for those, but you still have to create a forest of mocks.

Another point is what happens if you have navigation A.B but during the production you want to reverse its direction to B.A. Suddenly you’ll have to refactor all the code based on that navigation.

I know that is 10000x more difficult, but keeping the navigation internal, in the entities, and using a very strict ‘Tell don’t Ask’ will be on a long run more efficient.javascript:WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions(“ctl00$ctl00$ctl00$bcr$bcr$bcr$ctl11$ctl02$ctl02$ctl02$btnSubmit”, “”, true, “CreateCommentForm”, “”, false, true))
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By: Jimmy Bogard

Jimmy Bogard — Thu, 11 Sep 2008 11:22:04 +0000

@Tudor-Andrei

Do you have an example of this? Both Law of Demeter and how unit testing are made difficult?

By: Tudor-Andrei Pamula

Tudor-Andrei Pamula — Thu, 11 Sep 2008 06:47:58 +0000

Hi,
thank you for the very good explanation of these DDD basic concepts.
I still disagree with the following statement:
‘Only Aggregate Roots can be obtained directly with database queries. Everything else must be done through traversal.’

Navigation usually means exposing internal data (getters/setters) and leads to ‘Law of Demeter’ violation, and will make unit-testing more difficult
(see design-style/’train wrecks’ http://www.martinfowler.com/articles/mocksArentStubs.html)

By: Mark Holtman

Mark Holtman — Thu, 28 Aug 2008 13:38:43 +0000

Jimmy,

Thanks for a very concise explanation of these basic concepts. Your concrete examples helped solidify some of my thinking. I’m making my way through Evan’s DDD book at the moment and was struggling with what a Value Object is, ideally.