195 lines
6.8 KiB
Python
195 lines
6.8 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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from __future__ import unicode_literals
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from django.db.backends import util
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from django.utils import tree
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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 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 = type(self)()
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obj.add(self, conn)
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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 = type(self)()
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obj.add(self, self.AND)
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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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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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from django.db.models.fields import FieldDoesNotExist
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non_deferred_model = instance._meta.proxy_for_model
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opts = non_deferred_model._meta
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assert instance is not None
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data = instance.__dict__
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if data.get(self.field_name, self) is self:
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# self.field_name is the attname of the field, but only() takes the
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# actual name, so we need to translate it here.
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try:
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f = opts.get_field_by_name(self.field_name)[0]
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except FieldDoesNotExist:
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f = [f for f in opts.fields
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if f.attname == self.field_name][0]
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name = f.name
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# Lets see if the field is part of the parent chain. If so we
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# might be able to reuse the already loaded value. Refs #18343.
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val = self._check_parent_chain(instance, name)
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if val is None:
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# We use only() instead of values() here because we want the
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# various data coersion methods (to_python(), etc.) to be
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# called here.
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val = getattr(
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non_deferred_model._base_manager.only(name).using(
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instance._state.db).get(pk=instance.pk),
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self.field_name
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)
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data[self.field_name] = val
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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 _check_parent_chain(self, instance, name):
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"""
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Check if the field value can be fetched from a parent field already
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loaded in the instance. This can be done if the to-be fetched
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field is a primary key field.
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"""
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opts = instance._meta
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f = opts.get_field_by_name(name)[0]
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link_field = opts.get_ancestor_link(f.model)
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if f.primary_key and f != link_field:
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return getattr(instance, link_field.attname)
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return None
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def select_related_descend(field, restricted, requested, load_fields, 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_klass_info()).
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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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* load_fields - the set of fields to be loaded on this model
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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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if load_fields:
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if field.name not in load_fields:
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if restricted and field.name in requested:
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raise InvalidQuery("Field %s.%s cannot be both deferred"
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" and traversed using select_related"
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" at the same time." %
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(field.model._meta.object_name, field.name))
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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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proxy = True
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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 existing class
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# object if the attrs are identical.
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name = "%s_Deferred_%s" % (model.__name__, '_'.join(sorted(list(attrs))))
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name = util.truncate_name(name, 80, 32)
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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(str(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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