Note: Developers building new applications are strongly encouraged to use the NDB Client Library, which has several benefits compared to this client library, such as automatic entity caching via the Memcache API. If you are currently using the older DB Client Library, read the DB to NDB Migration Guide
Class Model
is the superclass for data model definitions.
Model
is defined in the module google.appengine.ext.db
.
Introduction
An application defines a data model by defining a class that subclasses
Model
. Properties of the model are defined using class attributes
and
Property
class instances. For example:
class Story(db.Model): title = db.StringProperty() body = db.TextProperty() created = db.DateTimeProperty(auto_now_add=True)
An application creates a new data entity by instantiating a subclass of the
Model
class. Properties of an entity can be assigned using attributes of the instance,
or as keyword arguments to the constructor.
s = Story() s.title = "The Three Little Pigs" s = Story(title="The Three Little Pigs")
The name of the model sub-class is used as the name of the Datastore entity
kind. The Datastore reserves all kind names that begin with two underscores
(__
). Model sub-classes must not use such names.
The names of the attributes are used as the names of the corresponding
properties on an entity. Model instance attributes whose names begin with an
underscore (_
) are ignored, so your application can use such
attributes to store data on a model instance that isn't saved to the
Datastore.
The Datastore and the model class API impose several restrictions on property names and model instance attributes. See Disallowed Property Names for a complete description.
Every entity has a
key,
a unique identifier that represents the entity. The key can include an optional
key name, a string unique across entities of the given kind. The
entity's kind and key name can be used with the
Key.from_path()
and
Model.get_by_key_name()
methods to retrieve the entity.
An entity can also have an optional
parent
entity. Parent-child relationships form
entity groups,
which are used to control transactionality and data locality in the Datastore.
An application creates a parent-child relationship between two entities by
passing the parent entity to the child entity's constructor, as the
parent
argument.
The method
Model.get_or_insert()
can be used to retrieve an entity that may not exist, creating it in the
Datastore if necessary:
keyname = "some_key" s = Story.get_or_insert(keyname, title="The Three Little Pigs")
Note: A model instance does not have a corresponding entity in the Datastore until it is written
(put)
for the first time, either explicitly or via
Model.get_or_insert()
.
To create a dict
that is a copy of a model instance's data, use
the
db.to_dict
function.
Constructor
The constructor for class Model
is defined as follows:
- class Model (parent=None, key_name=None, **kwds)
-
The superclass for data model definitions.
During construction, each property's
validate()
method is called. Exceptions from such calls propagate to callers of this constructor.Arguments
- parent
- The model instance or key for the entity that is the new entity's parent.
- key_name
-
The key name for the entity. The name becomes part of the primary key. If
None
, a system-generated numeric ID is used for the key.The value for
key_name
must not be of the form__*__
.The key name is stored as a Unicode string, with
str
values converted as ASCII text.Calling
put()
on this object will overwrite any existing Datastore entity with the same key. - kwds
- Initial values for the instance's properties, as keyword arguments. Each name corresponds with an attribute defined on the Model class.
Additional keyword argument
- key
-
The explicit
Key
instance for the entity. Cannot be used withkey_name
orparent
. IfNone
, falls back on the behavior forkey_name
andparent
. Useful when usingallocate_ids()
to reserve numeric IDs for new entities.The value for
key
must be a validKey
instance.Calling
put()
on this object will overwrite any existing Datastore entity with the same key.
Class Methods
Class Model
has the following class methods:
- Model.get (keys)
-
Retrieves the model instance (or instances) for the given key (or keys). The keys must represent entities of the model's kind. If a provided key is not of the correct kind, a
KindError
exception is raised.This method is similar to the
db.get()
function, with additional type checking.Arguments
- keys
- Key of entity to be retrieved, a string representation of the key, or a list of keys or their string representations.
- read_policy
- Read policy specifying desired level of data consistency:
- STRONG_CONSISTENCY
- Guarantees the freshest results, but limited to a single entity group.
- EVENTUAL_CONSISTENCY
- Can span multiple entity groups, but may occasionally return stale results. In general, eventually consistent queries run faster than strongly consistent queries, but there is no guarantee.
Note: Global (non-ancestor) queries ignore this argument.
- deadline
- Maximum time, in seconds, to wait for Datastore to return a result before aborting with an error. Accepts either an integer or a floating-point value. Cannot be set higher than the default value (60 seconds), but can be adjusted downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user, retry the operation, try a different operation, or add the operation to a task queue).
If
keys
consists of a single key (or its string representation), this method returns the model instance associated with the key if the key exists in the Datastore, otherwiseNone
. Ifkeys
is a list, the return value is a corresponding list of model instances, withNone
values where no entity exists for a given key.See also the
db.get()
function. - Model.get_by_id (ids, parent=None)
-
Retrieves the model instance (or instances) for the given numeric ID (or IDs).
Arguments
- ids
- A numeric entity ID, or a list of numeric IDs.
- parent
- The parent entity for the requested entities, as a model or key, or
None
(the default) if the requested entities do not have a parent. Multiple entities requested by one call must all have the same parent. - read_policy
- Read policy specifying desired level of data consistency:
- STRONG_CONSISTENCY
- Guarantees the freshest results, but limited to a single entity group.
- EVENTUAL_CONSISTENCY
- Can span multiple entity groups, but may occasionally return stale results. In general, eventually consistent queries run faster than strongly consistent queries, but there is no guarantee.
Note: Global (non-ancestor) queries ignore this argument.
- deadline
- Maximum time, in seconds, to wait for Datastore to return a result before aborting with an error. Accepts either an integer or a floating-point value. Cannot be set higher than the default value (60 seconds), but can be adjusted downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user, retry the operation, try a different operation, or add the operation to a task queue).
If
ids
consists of a single numeric ID, this method returns the model instance associated with the ID if the ID exists in the Datastore, otherwiseNone
. Ifids
is a list, the return value is a corresponding list of model instances, withNone
values where no entity exists for a given numeric ID. - Model.get_by_key_name (key_names, parent=None)
-
Retrieves the model instance (or instances) for the given key name (or names).
Arguments
- key_names
- A key name, or a list of key names.
- parent
- The parent entity for the requested entities, as a model instance or
key, or
None
(the default) if the requested entities do not have a parent. Multiple entities requested by one call must all have the same parent. - read_policy
- Read policy specifying desired level of data consistency:
- STRONG_CONSISTENCY
- Guarantees the freshest results, but limited to a single entity group.
- EVENTUAL_CONSISTENCY
- Can span multiple entity groups, but may occasionally return stale results. In general, eventually consistent queries run faster than strongly consistent queries, but there is no guarantee.
Note: Global (non-ancestor) queries ignore this argument.
- deadline
- Maximum time, in seconds, to wait for Datastore to return a result before aborting with an error. Accepts either an integer or a floating-point value. Cannot be set higher than the default value (60 seconds), but can be adjusted downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user, retry the operation, try a different operation, or add the operation to a task queue).
If
key_names
consists of a single key name, this method returns the model instance associated with the name if the name exists in the Datastore, otherwiseNone
. Ifkey_names
is a list, the return value is a corresponding list of model instances, withNone
values where no entity exists for a given key name. - Model.get_or_insert (key_name, **kwds)
-
Attempts to get the entity of the model's kind with the given key name. If it exists,
get_or_insert()
simply returns it. If it doesn't exist, a new entity with the given kind, name, and parameters inkwds
is created, stored, and returned.The get and subsequent (possible) put operations are wrapped in a transaction to ensure atomicity. Ths means that
get_or_insert()
will never overwrite an existing entity, and will insert a new entity if and only if no entity with the given kind and name exists. In other words,get_or_insert()
is equivalent to the following Python code:def txn(key_name, **kwds): entity = Story.get_by_key_name(key_name, parent=kwds.get('parent')) if entity is None: entity = Story(key_name=key_name, **kwds) entity.put() return entity def get_or_insert(key_name, **kwargs): return db.run_in_transaction(txn, key_name, **kwargs) get_or_insert('some key', title="The Three Little Pigs")
Arguments
- key_name
- The name for the key of the entity
- kwds
- Keyword arguments to pass to the model class's constructor if an
instance with the specified key name doesn't exist. The
parent
argument is required if the desired entity has a parent.
Note:
get_or_insert()
does not accept aread_policy
ordeadline
argument.The method returns an instance of the model class that represents the requested entity, whether it existed or was created by the method. As with all Datastore operations, this method can raise a
TransactionFailedError
if the transaction could not be completed. - Model.all (keys_only=False)
-
Returns a
Query
object that represents all entities for the kind corresponding to this model. Methods on the query object can apply filters and sort orders to the query before it is executed; see theQuery
class page for more information.Arguments
- keys_only
- Whether the query should return full entities or just keys. Queries that return keys are faster and use less CPU time than queries that return full entities.
- Model.gql (query_string, *args, **kwds)
-
Performs a GQL query over instances of this model.
Arguments
- query_string
- The part of the GQL query following
SELECT
*
FROM
model
(which is implied by using this class method). - args
- Positional parameter bindings, similar to the
GqlQuery()
constructor. - kwds
- Keyword parameter bindings, similar to the
GqlQuery()
constructor.
s = Story.gql("WHERE title = :1", "Little Red Riding Hood") s = Story.gql("WHERE title = :title", title="Little Red Riding Hood")
The return value is a
GqlQuery
object, which can be used to access the results. - Model.kind ()
- Returns the kind of the model, usually the name of the Model subclass.
- Model.properties ()
- Returns a dictionary of all of the properties defined for this model class.
Instance Methods
Model instances have the following methods:
- key ()
-
Returns the Datastore
Key
for this model instance.A model instance's key includes the instance's entity kind along with a unique identifier. The identifier may be either a key name string, assigned explicitly by the application when the instance is created, or an integer numeric ID, assigned automatically by App Engine when the instance is written (put) to the Datastore. Calling
key()
before the instance has been assigned an identifier raises aNotSavedError
exception. - put ()
-
Stores the model instance in the Datastore. If the model instance is newly created and has never been stored, this method creates a new data entity in the Datastore. Otherwise, it updates the data entity with the current property values.
The method returns the key of the stored entity.
If the data could not be committed, raises a
TransactionFailedError
exception.Arguments
- deadline
- Maximum time, in seconds, to wait for Datastore to return a result before aborting with an error. Accepts either an integer or a floating-point value. Cannot be set higher than the default value (60 seconds), but can be adjusted downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user, retry the operation, try a different operation, or add the operation to a task queue).
- delete ()
-
Deletes the model instance from the Datastore. If the instance has never been written (put) to the Datastore, the delete raises a
NotSavedError
exception.Arguments
- deadline
- Maximum time, in seconds, to wait for Datastore to return a result before aborting with an error. Accepts either an integer or a floating-point value. Cannot be set higher than the default value (60 seconds), but can be adjusted downward to ensure that a particular operation fails quickly (for instance, to return a faster response to the user, retry the operation, try a different operation, or add the operation to a task queue).
- is_saved ()
-
Returns
True
if the model instance has been written (put) to the Datastore at least once.This method only checks that the instance has been written to the Datastore at least once since it was created. It does not check whether the instance's properties have been updated since the last time it was written.
- dynamic_properties ()
-
Returns a list of the names of all of the dynamic properties defined for this model instance. This only applies to instances of
Expando
classes. For non-Expando model instances, this returns an empty list. - parent ()
-
Returns a model instance for the parent entity of this instance, or
None
if this instance does not have a parent. - parent_key ()
-
Returns the
Key
of the parent entity of this instance, orNone
if this instance does not have a parent. - to_xml ()
-
Returns an XML representation of the model instance.
Property values conform to the Atom and Data specifications.
Disallowed Property Names
The Datastore and its API impose several restrictions on names for entity properties and model instance attributes.
The Datastore reserves all property names that begin and end with two
underscores (__*__
). A Datastore entity cannot have a property with
such a name.
The Python model API ignores all attributes on a Model
or
Expando
class that begin with an underscore (_
). Your application can use
these attributes to associate data with the model objects that is not saved to
the Datastore.
Lastly, the Python model API uses object attributes to define properties of a
model, and by default the Datastore entity properties are named after the
attributes. Because the Model
class has several properties and
methods for other purposes, those attributes cannot be used for properties in
the Python API. For example, a Model cannot have a property accessed with the
attribute key
.
However, a property can specify a different name for the Datastore than the
attribute name by giving a name
argument to the property
constructor. This allows the Datastore entity to have a property name similar to
a reserved attribute in the Model
class, and use a different
attribute name in the class.
class MyModel(db.Model): obj_key = db.StringProperty(name="key")
The following attribute names are reserved by the Model
class in
the Python API:
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