DDD valueObject and database schema

Question

To end 2014 year I got a simple question I think. I would like to use "DDD" a bit more, and I'm currently trying to experiment various usecases to learn more about DDD.

My current usecase is the following :

• we have a new database schema that is using a classic pattern in our company : modeling our nomenclature table as "id / code / label". I think it's a pretty classic case when using hibernate for example.

But in the OO world things get "complciated" for something this simple when using a API like JDBC or QueryDSL. I need to fetch an object by its code, retrieve its id or load the full object and then set it as a one to one relation in another object.

I wondering :

• this kind of nomenclature can be an enum (or a class with String cosnatnts depending on the developer). in DDD terms, it is my ValueObject
• the id  /code / label in the database is not i18n friendly (it's not a prerequisite) so I don't see its advantages. Except when the table can be updated dynamically and the usecase is "pick something in a combobox loaded from this table and build a relation with another object : but that's all because if you have business rules that must be applied you need to know the new code etc etc).

My questions are :

• do you often use the id / ocde / label pattern in your database model.
• how do your model your nomenclature data ? (country is perhaps not the best example :) but no matter what how do you model it ? without thinking much I would say database table for country; but for some status : "valid, waiting validation, rejected" ?
• do you model your valueObjects using this pattern ?
• or do you use lots of enum and only store their toString (or ordinal) in the database ?

In the Java OO objects world, I'm currently thinking that it is easier to manipulate enum that objects loaded from the database. I need to build repositories to load them for example. And it will be so simple to use them as enums. I'm searching some recomfort here or perhaps am I missing something so obvious ?

thanks

see you in 2015 !

Update 1 : We can create a "Budget" and the first one is mark as Initial and the next ones are marked as "Corrective" (with a increment). For example, we can have a list of Budgets :"Initial Budget", "Corrective budget #1", "Corrective budget #2".

For this we have this database design : a Budget Table, a Version Budge with a foreign key between the two. the Version budget only contains an ID, a CODE and a LABEL.

Personnaly, I would like to remove this table. I don't see the advantages of this structure. And from the OO perspective, when I'm creating a budget I can query the databse to see if I need to create an Inital or Corrective budget (using a count query) then I can set the right enum to my new budget. But with the current design I need to query the database using the CODE that I want, select the ID and set the ID. So yes, it's really database oriented. Where is the DDD part ? a ValueObject is something that describe, quantify something. In my case seems good to me. A Version describe the current status of my Budget. I can comapre two versions just but checking their code, they don't have lifecycle (I don't want this one in particular).

How to you handle this type of usecases ?

It's only a simple example because I found that if you ask a database admin he would surely said that all seems good : using primary key, modeling relations, enforing constraints, using foreign key and avoid data duplication.

Thanks again Mike and Doctor for their comments.

Answer 1

I will hook in in your country example. In most cases, country will be a value object. There is nothing that will reference a country entity and that should know that if the values of the country changes it is still the same country. In fact, the country could be represented as an enum, and some nasty resource lookup functions that translate the Iso3 into a usefull display text. What we do is, we define it as a value object class with iso3, displayname and some other static information. Now out of this value object we define a kind of "power enum" (I still miss a standard term here). The class implementing the country value object gets a private constructor and static properties for each of its values (for each country) and explicit cast operators from and to int. Now you can treat it just like a normal enum of your programing language. The advantage to a normal enum beside having more property fields is, that it also can have methods (of course query methods, that don't change the state of the object). You can even use polymorphism (some countries with different behaviour than others). You could also load the content of the enums from a database table (without the statics then and a static lookupByIso3 method instead).

This you could make with some other "enum like" value objects, too. Imagine Currencies (it could have conversion methods that are implemented polymorphic). The handling of the daily exchange rates is a different topic though.

If the set of values is not fixed (for example another value object candidate like postal adress) then it is not a value object enum, but a standard value object that could be instantiated with the values you want.

To decide if you can live with something as a value object, you can use the following question: Do you want copy semantic, or reference semantic? If you ever change a property of the object, should all places where you used it update, too, or should they stay as they are? If the latter, than the "changed" object is a new and different value object. Another question would be, if you need to track changes to an object realizing that it remains the "same" despite of changing values. And if you have a value object, where you only want specific instances to exist, it is a kind of enum described above.

Does that somehow help you?