257 lines
8.3 KiB
Python
257 lines
8.3 KiB
Python
"""
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Various data structures used in query construction.
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Factored out from django.db.models.query to avoid making the main module very
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large and/or so that they can be used by other modules without getting into
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circular import difficulties.
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"""
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import weakref
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from django.utils.copycompat import deepcopy
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from django.utils import tree
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from django.utils.datastructures import SortedDict
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class CyclicDependency(Exception):
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"""
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An error when dealing with a collection of objects that have a cyclic
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dependency, i.e. when deleting multiple objects.
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"""
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pass
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class InvalidQuery(Exception):
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"""
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The query passed to raw isn't a safe query to use with raw.
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"""
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pass
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class CollectedObjects(object):
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"""
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A container that stores keys and lists of values along with remembering the
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parent objects for all the keys.
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This is used for the database object deletion routines so that we can
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calculate the 'leaf' objects which should be deleted first.
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previously_seen is an optional argument. It must be a CollectedObjects
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instance itself; any previously_seen collected object will be blocked from
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being added to this instance.
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"""
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def __init__(self, previously_seen=None):
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self.data = {}
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self.children = {}
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if previously_seen:
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self.blocked = previously_seen.blocked
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for cls, seen in previously_seen.data.items():
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self.blocked.setdefault(cls, SortedDict()).update(seen)
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else:
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self.blocked = {}
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def add(self, model, pk, obj, parent_model, nullable=False):
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"""
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Adds an item to the container.
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Arguments:
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* model - the class of the object being added.
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* pk - the primary key.
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* obj - the object itself.
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* parent_model - the model of the parent object that this object was
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reached through.
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* nullable - should be True if this relation is nullable.
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Returns True if the item already existed in the structure and
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False otherwise.
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"""
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if pk in self.blocked.get(model, {}):
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return True
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d = self.data.setdefault(model, SortedDict())
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retval = pk in d
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d[pk] = obj
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# Nullable relationships can be ignored -- they are nulled out before
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# deleting, and therefore do not affect the order in which objects
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# have to be deleted.
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if parent_model is not None and not nullable:
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self.children.setdefault(parent_model, []).append(model)
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return retval
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def __contains__(self, key):
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return self.data.__contains__(key)
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def __getitem__(self, key):
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return self.data[key]
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def __nonzero__(self):
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return bool(self.data)
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def iteritems(self):
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for k in self.ordered_keys():
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yield k, self[k]
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def items(self):
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return list(self.iteritems())
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def keys(self):
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return self.ordered_keys()
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def ordered_keys(self):
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"""
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Returns the models in the order that they should be dealt with (i.e.
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models with no dependencies first).
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"""
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dealt_with = SortedDict()
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# Start with items that have no children
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models = self.data.keys()
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while len(dealt_with) < len(models):
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found = False
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for model in models:
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if model in dealt_with:
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continue
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children = self.children.setdefault(model, [])
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if len([c for c in children if c not in dealt_with]) == 0:
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dealt_with[model] = None
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found = True
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if not found:
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raise CyclicDependency(
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"There is a cyclic dependency of items to be processed.")
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return dealt_with.keys()
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def unordered_keys(self):
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"""
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Fallback for the case where is a cyclic dependency but we don't care.
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"""
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return self.data.keys()
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class QueryWrapper(object):
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"""
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A type that indicates the contents are an SQL fragment and the associate
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parameters. Can be used to pass opaque data to a where-clause, for example.
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"""
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def __init__(self, sql, params):
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self.data = sql, params
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def as_sql(self, qn=None, connection=None):
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return self.data
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class Q(tree.Node):
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"""
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Encapsulates filters as objects that can then be combined logically (using
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& and |).
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"""
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# Connection types
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AND = 'AND'
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OR = 'OR'
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default = AND
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def __init__(self, *args, **kwargs):
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super(Q, self).__init__(children=list(args) + kwargs.items())
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def _combine(self, other, conn):
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if not isinstance(other, Q):
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raise TypeError(other)
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obj = deepcopy(self)
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obj.add(other, conn)
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return obj
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def __or__(self, other):
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return self._combine(other, self.OR)
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def __and__(self, other):
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return self._combine(other, self.AND)
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def __invert__(self):
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obj = deepcopy(self)
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obj.negate()
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return obj
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class DeferredAttribute(object):
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"""
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A wrapper for a deferred-loading field. When the value is read from this
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object the first time, the query is executed.
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"""
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def __init__(self, field_name, model):
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self.field_name = field_name
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self.model_ref = weakref.ref(model)
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self.loaded = False
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def __get__(self, instance, owner):
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"""
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Retrieves and caches the value from the datastore on the first lookup.
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Returns the cached value.
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"""
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assert instance is not None
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cls = self.model_ref()
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data = instance.__dict__
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if data.get(self.field_name, self) is self:
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data[self.field_name] = cls._base_manager.filter(pk=instance.pk).values_list(self.field_name, flat=True).using(instance._state.db).get()
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return data[self.field_name]
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def __set__(self, instance, value):
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"""
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Deferred loading attributes can be set normally (which means there will
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never be a database lookup involved.
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"""
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instance.__dict__[self.field_name] = value
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def select_related_descend(field, restricted, requested, reverse=False):
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"""
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Returns True if this field should be used to descend deeper for
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select_related() purposes. Used by both the query construction code
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(sql.query.fill_related_selections()) and the model instance creation code
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(query.get_cached_row()).
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Arguments:
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* field - the field to be checked
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* restricted - a boolean field, indicating if the field list has been
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manually restricted using a requested clause)
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* requested - The select_related() dictionary.
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* reverse - boolean, True if we are checking a reverse select related
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"""
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if not field.rel:
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return False
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if field.rel.parent_link and not reverse:
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return False
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if restricted:
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if reverse and field.related_query_name() not in requested:
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return False
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if not reverse and field.name not in requested:
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return False
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if not restricted and field.null:
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return False
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return True
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# This function is needed because data descriptors must be defined on a class
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# object, not an instance, to have any effect.
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def deferred_class_factory(model, attrs):
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"""
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Returns a class object that is a copy of "model" with the specified "attrs"
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being replaced with DeferredAttribute objects. The "pk_value" ties the
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deferred attributes to a particular instance of the model.
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"""
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class Meta:
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pass
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setattr(Meta, "proxy", True)
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setattr(Meta, "app_label", model._meta.app_label)
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# The app_cache wants a unique name for each model, otherwise the new class
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# won't be created (we get an old one back). Therefore, we generate the
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# name using the passed in attrs. It's OK to reuse an old case if the attrs
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# are identical.
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name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))
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overrides = dict([(attr, DeferredAttribute(attr, model))
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for attr in attrs])
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overrides["Meta"] = Meta
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overrides["__module__"] = model.__module__
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overrides["_deferred"] = True
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return type(name, (model,), overrides)
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# The above function is also used to unpickle model instances with deferred
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# fields.
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deferred_class_factory.__safe_for_unpickling__ = True
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