[TYPO3-mvc] Persistence of value objects

[Jochen Rau]

Hi,

I would like to propose a more radical approach: What if we drop the
whole distinction between Entity and ValueObjects?

Recently I spent some time thinking about the difference between these
two DDD building blocks. I came to the conclusion that there is none.
Eric Evans argues from a technical standpoint. The promise is, to
differentiate between Entities and ValueObjects will lead to a
performance gain and a simpler system. In fact it is quite the opposite.
And from a Domain perspective, we don't need to make such a distinction.

Let's take the color "red" for example. It doesn’t matter which "red" we
assign to a product. It's always "red". So, we create a ValueObject with
the property $value="red".

Following Eric Evans, we don't assign an intrinsic identity to the
ValueObject. Instead we take the hash over all property values.

Now the problems arise. We want to output the color in French instead of
English. Is the color "rouge" different from "red". Well, yes, uhm, no.
In fact there are two things here: the concept [1] of red and the
different representations of red ("red", "rouge", "#f00", etc.).  In my
opinion, the abstract concept and the different representations are all
Entities (with their intrinsic identity). The only difference is, that
the representations refer to the abstract concept. If we want to assign
a color to a product we should always assign the concept to it, not a
representation. If we serialize the product to a representation (say in
HTML) we determine the best representation for the given context (format
HTML, current date and time, language, etc.).

Color is probably a bad example. How about an address, a number, a name?
I found no difference here. It's just the wrong way to cut the Domain
into two. The line to draw is between abstract concepts and their
concrete representations (aka non-information resources and their
representations).

So, I propose to deprecate the use of ValueObjects. In addition, we
should come up with some guide how to model the domain with the
distinction between concept and representation in mind.

What do you think?

Best
Jochen

[1] There is a joke about the concept of Jochen. I don't like it.

On Wed, Apr 18, 2012 at 03:07:51PM +0200, Dennis Ahrens wrote:
> Hi list,
> 
> i currently try to change my domain model for a person. Therefore
> i'd like to make the assigned addresses a value object, because
> there are many instances with the same address and i don't care
> about all existing addresses.
> 
> When monitoring the database i've noticed that same addresses were
> saved as seperate instances, with a uid etc.
> 
> To verfiy this, i've installed the blog_example and had a look at
> how tags are persisted. It behaves in the same way.
> 
> It looks as if they are handled in the same way like entities, is
> there a reason for this behaviour? I guess this is not the intended
> way, is it?
> 
> I'm currently using extbase 1.4.1 with some cherry picks, which
> should not affect this part of persistence.
> 
> regards
> Dennis