google.appengine.ext.db.Reference

A property that represents a many-to-one reference to another model.

Inherits From: Property, expected_type

google.appengine.ext.db.Reference(
    reference_class=None, verbose_name=None, collection_name=None, **attrs
)

For example, a reference property in model A that refers to model B forms a many-to-one relationship from A to B: every instance of A refers to a single B instance, and every B instance can have many A instances refer to it.

reference_class Which model class this property references.
verbose_name User friendly name of property.
collection_name If provided, alternate name of collection on reference_class to store back references. Use this to allow a Model to have multiple fields which refer to the same class.

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.

Returns
Default value as provided by __init__(default).

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.

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

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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.

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

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get_value_for_datastore(
    model_instance
)

Get key of reference rather than reference itself.

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.

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

View source

make_value_from_datastore_index_value(
    index_value
)

validate

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validate(
    value
)

Validate reference.

Returns
A valid value.

Raises
BadValueError for the following reasons:

  • Value is not saved.
  • Object not of correct model type for reference.

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