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"""
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.db.backends import util
from django.utils import tree
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
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