Value object depends on other value object

Question

As I understand, most of the business logic is stored in the value objects as constraints, like the price cannot be less than 0.

But I have a problem where the value object depends on another value object as a constraint.

Says we have 2 value objects BasePrice and Price in the Product entity. Both the BasePrice and Price could be set separately. But the Price cannot be, says, greater than BasePrice + 1000.

Since both of them can be set separately, if we update the BasePrice, the Price might be not valid anymore.

How to solve these dependencies between value objects?

Answer 1

Not all constraints can be validated within a value object.

If the constraint is about the relation between two VOs then the constraint has to be enforced by something that holds them both. In DDD this could be a third value object or an entity. In this case it sounds like that would be the Product.

That object can throw throw an exception if there's an attempt to create an invalid combination of Price and BasePrice. The answer by Doc Brown shows a nice way of doing that.

Answer 2

A Price object which depends on an updateable BasePrice cannot be a value object, since value objects should be immutable (at least, by the book).

So if Price and BasePrice are both immutable, and Price holds a reference to a BasePrice, if one needs a price with a different base price, it will be necessary to create a new Price object with a new BasePrice passed in the constructor, so the old Price object stays valid. However, if Price and BasePrice should both be properties of a Product, this is probably not a good idea, since now the Price object of the product might reference a different base price than the Product itself.

Hence, when you want Price and BasePrice to be independent properties of your Product, each one should not know anything about the other directly. Therefore, the constraint "Price must be between BasePrice and BasePrice+1000" makes sense only in the context of a Product object. This constraint needs to be checked whenever a method like Product.SetPrice or Product.SetBasePrice is called.

Still, the business logic for checking against the base price can be part of the Price object. Design the latter with a method IsInValidRange(BasePrice bp), and call it like this

   class Product
   {
        BasePrice basePrice;
        Price price;
        
        void ChangePrice(Price newPrice)
        {
             if(!newPrice.IsInValidRange(basePrice))
                  throw new InvalidPriceException();
             price = newPrice;
        }

        void ChangeBasePrice(BasePrice newBasePrice)
        {
             if(!price.IsInValidRange(newBasePrice))
                  throw new InvalidPriceException();
             basePrice = newBasePrice;
        }
   }

I guess your issue lies in the the phrase "business logic stored in the value objects as constraints" - better replace "stored" by "implemented", this yields to the kind of solution I sketched above.

Answer 3

It sounds like you have a thing object that has two price values, not two separate objects. Since they have a dependency on each other it doesn't make sense to try and make the separate. From there your options are managing that relation in getter and setter methods of properties. In this case I would probably advise using explicit methods rather than something like a traditional .Net property, because there are side effects to changing these values that can be more clear with a method like thing.updatePricing(5, thing.Price) than thing.BasePrice=5;.