How to associate calculated values to an object

I have some basic objects like Customer, Portfolio and ... with some association to other objects. I can easily display the required information in the web page by reading object values. The problem is that what do I do when the value associated with the object is calculated and returned by a method, a value that makes sense only in certain context and cannot be attached to the object as an instance variable? In this case if I have a list of say Users I have to pass the username of each user to the method to get the calculated value. This causes problem to keep the association while displaying the values in the page.

An example to make this clear:

An application provides the functionality for users to keep track of each others activities by letting them add whoever they want to a list. If this user performs a search on users there's the option to follow each returned user. I want to make sure this option is disabled for those user's that are already being followed. This functionality is provided by a method like isFollowed(String follower, String followee) which returnes a boolean. How can I associate this boolean values to each user in search result?

Solutions:

One thing I can think of is to add a followed instance variable to User class. But I don't think it's a good approach because this variable only makes sense in a certain context. It's not a part of User class in the domain.

The other way I can think of is to use Decoration or Wrappers in a way to extend the User class and add the attribute in the child class. But again what if I have several objects that need to 开发者_运维百科be in the same context. In that case I have to extend all of them with the same boolean attribute in all classes.

I hope I could make it clear.

In principle, I don't see anything wrong with instance method on User: bool IsFollowedBy(User user).

Of course, this could lead to performance issues. If that is the case, you can create separate object for presentation purposes which bundles data from User and whether he is being followed by the user performing search. Then you can build query which retrieves all necessary data for such object in a single roundtrip to DB.

One solution is to avoid querying Entities (as in DDD/ORM) and query directly using subquery/join or even using some denormalized database. This is something CQRS pattern suggests.

Other solution is to do computations on application layer (how many Users can you show on the same page anyway), which is expensive but you can implement some caching techniques to make things easier.