query_utils.py

"""
Various data structures used in query construction.

Factored out from django.db.models.query to avoid making the main module very
large and/or so that they can be used by other modules without getting into
circular import difficulties.
"""

import weakref
from django.utils.copycompat import deepcopy

from django.db.backends import util
from django.utils import tree
from django.utils.datastructures import SortedDict


class InvalidQuery(Exception):
    """
    The query passed to raw isn't a safe query to use with raw.
    """
    pass


class QueryWrapper(object):
    """
    A type that indicates the contents are an SQL fragment and the associate
    parameters. Can be used to pass opaque data to a where-clause, for example.
    """
    def __init__(self, sql, params):
        self.data = sql, params

    def as_sql(self, qn=None, connection=None):
        return self.data

class Q(tree.Node):
    """
    Encapsulates filters as objects that can then be combined logically (using
    & and |).
    """
    # Connection types
    AND = 'AND'
    OR = 'OR'
    default = AND

    def __init__(self, *args, **kwargs):
        super(Q, self).__init__(children=list(args) + kwargs.items())

    def _combine(self, other, conn):
        if not isinstance(other, Q):
            raise TypeError(other)
        obj = type(self)()
        obj.add(self, conn)
        obj.add(other, conn)
        return obj

    def __or__(self, other):
        return self._combine(other, self.OR)

    def __and__(self, other):
        return self._combine(other, self.AND)

    def __invert__(self):
        obj = type(self)()
        obj.add(self, self.AND)
        obj.negate()
        return obj

class DeferredAttribute(object):
    """
    A wrapper for a deferred-loading field. When the value is read from this
    object the first time, the query is executed.
    """
    def __init__(self, field_name, model):
        self.field_name = field_name
        self.model_ref = weakref.ref(model)
        self.loaded = False

    def __get__(self, instance, owner):
        """
        Retrieves and caches the value from the datastore on the first lookup.
        Returns the cached value.
        """
        from django.db.models.fields import FieldDoesNotExist

        assert instance is not None
        cls = self.model_ref()
        data = instance.__dict__
        if data.get(self.field_name, self) is self:
            # self.field_name is the attname of the field, but only() takes the
            # actual name, so we need to translate it here.
            try:
                cls._meta.get_field_by_name(self.field_name)
                name = self.field_name
            except FieldDoesNotExist:
                name = [f.name for f in cls._meta.fields
                    if f.attname == self.field_name][0]
            # We use only() instead of values() here because we want the
            # various data coersion methods (to_python(), etc.) to be called
            # here.
            val = getattr(
                cls._base_manager.filter(pk=instance.pk).only(name).using(
                    instance._state.db).get(),
                self.field_name
            )
            data[self.field_name] = val
        return data[self.field_name]

    def __set__(self, instance, value):
        """
        Deferred loading attributes can be set normally (which means there will
        never be a database lookup involved.
        """
        instance.__dict__[self.field_name] = value

def select_related_descend(field, restricted, requested, reverse=False):
    """
    Returns True if this field should be used to descend deeper for
    select_related() purposes. Used by both the query construction code
    (sql.query.fill_related_selections()) and the model instance creation code
    (query.get_cached_row()).

    Arguments:
     * field - the field to be checked
     * restricted - a boolean field, indicating if the field list has been
       manually restricted using a requested clause)
     * requested - The select_related() dictionary.
     * reverse - boolean, True if we are checking a reverse select related
    """
    if not field.rel:
        return False
    if field.rel.parent_link and not reverse:
        return False
    if restricted:
        if reverse and field.related_query_name() not in requested:
            return False
        if not reverse and field.name not in requested:
            return False
    if not restricted and field.null:
        return False
    return True

# This function is needed because data descriptors must be defined on a class
# object, not an instance, to have any effect.

def deferred_class_factory(model, attrs):
    """
    Returns a class object that is a copy of "model" with the specified "attrs"
    being replaced with DeferredAttribute objects. The "pk_value" ties the
    deferred attributes to a particular instance of the model.
    """
    class Meta:
        proxy = True
        app_label = model._meta.app_label

    # The app_cache wants a unique name for each model, otherwise the new class
    # won't be created (we get an old one back). Therefore, we generate the
    # name using the passed in attrs. It's OK to reuse an existing class
    # object if the attrs are identical.
    name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))
    name = util.truncate_name(name, 80, 32)

    overrides = dict([(attr, DeferredAttribute(attr, model))
            for attr in attrs])
    overrides["Meta"] = Meta
    overrides["__module__"] = model.__module__
    overrides["_deferred"] = True
    return type(name, (model,), overrides)

# The above function is also used to unpickle model instances with deferred
# fields.
deferred_class_factory.__safe_for_unpickling__ = True