You often need subsets of objects in a collection and want to access them efficiently in your domain model. But you certainly don't want to access the EntityManager or any other object manager here to craft a query. FilterExpressions for collections allow to go back to the database and query for all objects matching the crafted expression. Additionally they also work against in meemory ArrayCollection exactly the same. This way you don't (except for the SQL performance when it haunts you ;)) have to think about the context and can focus on your domain logic.
In Doctrine ORM this will be done by building DQL under the hood, in memory it will be done using Collection#filter(Closure $closure);
- Should allow filtering depending on the "persistence" backend, i.e. in memory for Arraycollection and using sql for PersistentCollection
- Should be very simple to be adoptable in many persistence providers
- Are always either accepting "Expr op Expr" xor "Field op value". A new Expression Language is needed for that, cannot reuse the ORM one.
- Assumes that for "Field" a getter "getField" exists on target object and that the field is mapped in any corresponding persistence provider.