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Property used for creating properties derived from other values.

Inherits From: Property, expected_type

google.appengine.ext.db.ComputedProperty(
    value_function, indexed=True
)

Certain attributes should never be set by users but automatically calculated at run-time from other values of the same entity. These values are implemented as persistent properties because they provide useful search keys.

A computed property behaves the same as normal properties except that you may not set values on them. Attempting to do so raises db.DerivedPropertyError which db.Model knows to ignore during entity loading time. Whenever getattr is used for the property the value is recalculated. This happens when the model calls get_value_for_datastore on the property.

Example:

import string

class Person(Model):

name = StringProperty(required=True)

@db.ComputedProperty def lower_case_name(self): return self.name.lower()

Find all people regardless of case used in name.

Person.gql('WHERE lower_case_name=:1' % name_to_search_for.lower())

Methods

datastore_type

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datastore_type()

Deprecated backwards-compatible accessor method for self.data_type.

default_value

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default_value()

Default value for unassigned values.

empty

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empty(
    value
)

Determine if value is empty in the context of this property.

For most kinds, this is equivalent to "not value", but for kinds like bool, the test is more subtle, so subclasses can override this method if necessary.

get_updated_value_for_datastore

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get_updated_value_for_datastore(
    model_instance
)

Determine new value for auto-updated property.

Some properies (e.g. DateTimeProperty, UserProperty) optionally update their value on every put(). This call must return the new desired value for such properties. For all other properties, this call must return AUTO_UPDATE_UNCHANGED.

get_value_for_datastore

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get_value_for_datastore(
    model_instance
)

Datastore representation of this property.

Looks for this property in the given model instance, and returns the proper datastore representation of the value that can be stored in a datastore entity. Most critically, it will fetch the datastore key value for reference properties.

Some properies (e.g. DateTimeProperty, UserProperty) optionally update their value on every put(). This call must return the current value for such properties (get_updated_value_for_datastore returns the new value).

make_value_from_datastore

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make_value_from_datastore(
    value
)

Native representation of this property.

Given a value retrieved from a datastore entity, return a value, possibly converted, to be stored on the model instance. Usually this returns the value unchanged, but a property class may override this when it uses a different datatype on the model instance than on the entity.

This API is not quite symmetric with get_value_for_datastore(), because the model instance on which to store the converted value may not exist yet -- we may be collecting values to be passed to a model constructor.

make_value_from_datastore_index_value

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make_value_from_datastore_index_value(
    index_value
)

validate

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validate(
    value
)

Assert that provided value is compatible with this property.