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Recently I was faced a question about whether a simple calculation should be put in the Entity layer, or should the Entity be pure for just storing the raw data and leave the calculation logics in the business layer.
So my question is whether it is sensible to encapsulate simple calculations in the properties in an entity class?
It depends on the type of architecture you want.
This would mean that an Order has a property (or method) that would return the total price of the order based on the OrderLines. The Order would also have a method AddOrderItem(Product product, int amount) and the Order would check if there is already an OrderLine for that specific product.
In such a model you would also have objects that are not real entities, like a Repository for accessing data or a Factory for creating entities. These are called Domain Services. An Application Layer is responsible for calling the Domain Services (for example to retrieve an entity from the database) and then it will execute functionality on the entity. The Application Layer should be as thin as possible.
This is a nice article about DDD which explains these concepts in more detail.
Order price and checking for duplicate OrderLines. There are different opinions whether an Anemic Domain Model is a bad thing. Personally I prefer a real Domain Model.
This article describes the differences between an Anemic and non-Anemic Domain Model.
Order class?
Well, Entity and business Objects are almost same, most of the time. For example if you have a product class and you want to expose a property which takes some existing property in product class and does some calculation and then exposes it. Its fine in the term that, the logic of creating that property remains with the class.
Now, the question may come, where to fit your business layer class. I prefer using business layer class which has some logic to deal with business problem. For example in your Product example, a business problem could be charging money using third party vendor like paypal.
One key thing to remember is an Entity would always have an identity but a business object are entity with out an identification. For example product is an entity but money would not have an identity. 1000 different instances of money would be same.