from django.db import connections
from django.db.models import Manager
from django.db.models.query import QuerySet
from django.db.models.sql.compiler import SQLCompiler
from django.db.models.sql.query import Query
from django.db.models.sql.where import WhereNode


class GroupedCompiler(SQLCompiler):
    def get_group_by(self, select, order_by):
        result = super(GroupedCompiler, self).get_group_by(select, order_by)
        expressions = []
        for expr in self.query.real_group_by:
            ref = expr if hasattr(expr, "as_sql") else self.query.resolve_ref(expr)
            sql, params = self.compile(ref)
            result.append((sql, params))

        return result


class GroupedQuery(Query):
    def __init__(self, model, where=WhereNode):
        super(GroupedQuery, self).__init__(model, where)
        self.real_group_by = []

    def clone(self, klass=None, memo=None, **kwargs):
        obj = super(GroupedQuery, self).clone(klass, memo, **kwargs)
        obj.real_group_by = self.real_group_by[:]
        return obj

    def add_grouping(self, *grouping):
        self.real_group_by.extend(grouping)

    def clear_grouping(self):
        self.real_group_by = []

    def get_compiler(self, using=None, connection=None):
        if using is None and connection is None:
            raise ValueError("Need either using or connection")
        if using:
            connection = connections[using]
        return GroupedCompiler(self, connection, using)


class GroupedQuerySet(QuerySet):
    def __init__(self, model=None, query=None, using=None, hints=None):
        super(GroupedQuerySet, self).__init__(model, query, using, hints)
        self.query = query or GroupedQuery(self.model)

    def group_by(self, *field_names):
        obj = self._clone()
        obj.query.clear_grouping()
        obj.query.add_grouping(*field_names)
        return obj


class GroupedManager(Manager):
    def __init__(self):
        super(GroupedManager, self).__init__()
        self._queryset_class = GroupedQuerySet