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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())
Args | |
|---|---|
value_function
|
Callable f(model_instance) -> value used to derive
persistent property value for storage in datastore.
|
indexed
|
Whether or not the attribute should be indexed. |
Methods
datastore_type
datastore_type()
Deprecated backwards-compatible accessor method for self.data_type.
default_value
default_value()
Default value for unassigned values.
| Returns | |
|---|---|
Default value as provided by __init__(default).
|
empty
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.
| Args | |
|---|---|
value
|
Value to validate against this Property.
|
| Returns | |
|---|---|
True if this value is considered empty in the context of this Property
type, otherwise False.
|
get_updated_value_for_datastore
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.
| Args | |
|---|---|
model_instance
|
Instance to get new value for. |
| Returns | |
|---|---|
Datastore representation of the new model value in a form that is
appropriate for storing in the datastore, or AUTO_UPDATE_UNCHANGED.
|
get_value_for_datastore
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).
| Args | |
|---|---|
model_instance
|
Instance to fetch datastore value from. |
| Returns | |
|---|---|
| Datastore representation of the model value in a form that is appropriate for storing in the datastore. |
make_value_from_datastore
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.
| Args | |
|---|---|
value
|
Value retrieved from the datastore entity. |
| Returns | |
|---|---|
| The value converted for use as a model instance attribute. |
make_value_from_datastore_index_value
make_value_from_datastore_index_value(
index_value
)
validate
validate(
value
)
Assert that provided value is compatible with this property.
| Args | |
|---|---|
value
|
Value to validate against this Property.
|
| Returns | |
|---|---|
| A valid value, either the input unchanged or adapted to the required type. |
| Raises | |
|---|---|
BadValueError if the value is not appropriate for this
property in any way.
|
Class Variables | |
|---|---|
| creation_counter |
2
|