Data: QueryChain

[lhaze]

It turned out that we need a query manipulation object, just like SQLAlchemy's Query & Django ORM's QuerySet. Predicate class is good at expressing an universal condition but there is more to do:

• Not all operaterations' signatures of DAO make sense to have all cases of filtering (eg. exists). Using query chaining like .filter(foo='bar').exists() make more sense.
• Sometimes you want to have final query built from a few predefined pieces (like having predefined user visibility queries and chaining them with a custom filter later).

[lhaze]

class SomeDao(IDao):
    def aggregate(*vars: str) -> QueryChain: ... 
    def sort(*vars: str) -> QueryChain: ... 
    def filter(self, predicate: Predicate) -> QueryChain: ...
        return QueryChain(self).filter(predicate)
    def _resolve_filter(self, predicate):
         return self.table
    def exists(self) -> bool: ...


class QueryChain:
    def __init__(self, dao):
    def aggregate(*vars: str) -> QuerySet: ... 
    def sort(*vars: str) -> QuerySet: ... 
    def filter(predicate: Predicate) -> QuerySet: ...

    def exists() -> bool: ... 

[lhaze]

Done.