Django Managers for pre-defined queries.

qmanager.py

# -*- coding: utf-8 -*-

"""
Managers for pre-defined queries.

Example::
    
    from django.db import models
    
    from qmanager import QManager
    
    class MyModel(models.Model):
        # Fields
        ...
        is_hidden = models.BooleanField()
        ...
        
        # Managers
        objects = models.Manager()
        public = QManager(models.Q(is_hidden=False))
        hidden = ~public # NOT public.

QManagers support combination via ``&`` and ``|``, with each other and also with
instances of ``django.db.models.Q`` (i.e. query objects).
"""

from django.db import models


def metaclass(cls):
    if not issubclass(cls, object):
        raise TypeError('Class %r does not support metaclasses.' % (cls,))
    return getattr(cls, '__metaclass__', type)


def get_query(obj):
    if (isinstance(obj, QManager) or
        (isinstance(obj, type) and issubclass(obj, QManager)) or
        hasattr(obj, 'query')):
        return obj.query
    elif isinstance(obj, models.Q):
        return obj
    raise TypeError('Object %r is not a QManager or Query object.' % (obj,))


class QManagerMeta(metaclass(models.Manager)):
    
    def __call__(cls, *args):
        if args:
            return cls.with_query(*args)()
        return super(QManagerMeta, cls).__call__()
    
    def with_query(cls, query):
        return type(cls.__name__, (cls,),
                    {'query': query, '__module__': cls.__module__})


class QManager(models.Manager):
    
    __metaclass__ = QManagerMeta
    query = models.Q()
    
    def __and__(self, other):
        return type(self)(self.query & get_query(other))
    
    def __or__(self, other):
        return type(self)(self.query | get_query(other))
    
    def __invert__(self):
        return type(self)(~self.query)
    
    def get_query_set(self):
        return super(QManager, self).get_query_set().filter(self.query)