Best practice for setting default values for model

问题:

What's the best way to set default properties for new entities in DDD? Also, what's the best way to set default states for complex properties (eg. collections)?

My feeling is that default values should be in the models themselves as they are a form of business rule ("by default, we want X's to be Y & Z"), and the domain represents the business. With this approach, maybe a static "GetNew()" method on the model itself would work:

public class Person {
    public string Name { get; set; }
    public DateTime DateOfBirth { get; set; }
    public bool IsAlive { get; set; }
    public List Limbs { get; set; }

    public static Person GetNew() {
        return new Person() { 
            IsAlive = true,
            Limbs = new List() { RightArm, LeftArm, RightLeg, LeftLeg }
        }
    }
}

Unfortunately in our case, we need the collection property to be set to all members of another list, and as this model is decoupled from its Repository/DbContext it doesn't have any way of loading them all.

Crappy solution would be to pass as parameter :

public static Person GetNew(List<Limb> allLimbs) {
    return new Person() { 
        IsAlive = true,
        Limbs = allLimbs
    }
}

Alternatively is there some better way of setting default values for simple & complex model properties?

回答1:

This is an instance of the factory pattern in DDD. It can either be a dedicated class, such as PersonFactory, or a static method, as in your example. I prefer the static method because I see no need to create a whole new class.

As far as initializing the collection, the GetNew method with the collection as a parameter is something I would go with. It states an important constraint - to create a new person entity you need that collection. The collection instance would be provided by an application service hosting the specific use case where it is needed. More generally, default values could be stored in the database, in which case the application service would call out to a repository to obtain the required values.

回答2:

Take a look at the Static Builder in Joshua Bloch's Effective Java (Second Edition). In there, you have a static builder class and you chain calls to set properties before construction so it solves the problem of either having a constructor that takes a ton of arguments or having to put setters on every property (in which case, you effectively have a Struct).