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Utilities for working with GAE NDB models

Project Description

Set of utilities for working with Google AppEngine Datastore.

The pckage currently has a few mixins for GAE NDB models, and a few custom properties.

NDB utils require FormEncode if you are using custom properties.


NDB utils can be installed from PyPI using easy_install:

easy_install ndb-utils

or with pip:

pip install ndb-utils

Model mixins

NDB utils comes with a few mixins for making common modelling tasks easier.


This mixin adds two properties to your models: created (creation timestamp), and updated (update timestamp). These are both ndb.DateTimeProperty with auto_now_add and auto_now respectively.


Mixin provides means for retrieving a random entity. Since retrieving all entites in order to pick a random item from the list would be too CPU intensive, this model mixin adds a random_id property, which contains a randomly generated integer which is used to look up ‘random’ entities. The random number is assigned in a _pre_put_hook.

During querying, it samples from the database a subset of 10 entities whose random_id is larger than another randomly generated integer and chooses a random entity from the sample.

This is obviously not a sure-fire way to retrieve a random entity. The random number generated during lookup may be larger than any of the random numbers in the database, so another query may be needed.

The mixin adds one classmethod used to retrieve a random entity: RandomMixin.random(). Calling this classmethod will retreive one random entity.

There is also a utility method RandomMixin.generate_random() which generates a random integer.


This mixin provides a method for checking uniqueness for a specified ancestry chain. This is best illustrated by example.

>>> from google.appengine.ext import ndb
>>> from ndb_utils.models import UniqueByAncestryMixin
>>> class Foo(ndb.Model):
...     prop = ndb.StringProperty()
>>> class Bar(ndb.Model):
...     prop = ndb.StringProperty()
>>> class Baz(UniqueByAncestryMixin, ndb.Model):
...     ancestry_path = ['Foo', 'Bar']
...     prop = ndb.StringProperty()
>>> foo = Foo(id='foo')
>>> bar = Bar(id='bar', parent=foo.key)
>>> baz = Baz(id='baz', parent=bar.key)
>>> ndb.put_multi([foo, bar, baz])
>>> Baz.is_unique('foo', 'bar', 'baz')

To break down the above, we define a model which specifies the ancestry path. The ancestry_path property is optional, and should be a list of ancestor kinds. The model’s own kind is implied, and will be appended to the ancestry path automatically.

The UniqueByAncestryMixin.is_unique() classmethod takes any number of positional arguments, which are interpolated into the ancestry path to build a pair of kind-id pairs used to build a key. In the example above, we are passing in (in order), the Foo’s id, then Bar’s id, and finally the Baz’s id, in order to test for uniqueness of the resulting key [('Foo', 'foo'), ('Bar', 'bar'), ('Baz', 'baz')].

The method returns a boolean that is True if the resulting key is not in use.

Note that this mixin is only useful if the id’s of all ancestors, as well as of the entity itself are known in advance. If your model uses an integer id provided by the datastore, you cannot use this mixin. (See the UniquePropertyMixin for an alternative solution.)

Usually, you want to check for uniqueness when creating a new entity. When doing so, note that no ancestor query is performed, and thus the is_unique() method cannot be used within transactions.

If you prefer to raise an exception when there is a clash, there is an exception class provided by NDB utils. You can raise this exception manually using the UniquePropertyMixin.DuplicateError class, or ndb_utils.exceptions.DuplicateError, or by calling the duplicate_error() classmethod passing it the same argument you passed to is_unique(). The last method is only a cosmetic thing. It provides a standard error message and nothing more.


This mixin provides methods for checking uniqueness of a set of properties across the datastore for a particular model. The properties that are to be checked are declared using the unique_properties class property.

Let’s take a look at an example:

>>> from google.appengine.ext import ndb
>>> from ndb_utils.models import UniquePropertyMixin
>>> class Foo(UniquePropertyMixin, ndb.Model):
...     unique_properties = ['prop']
...     prop = ndb.StringProperty()
>>> foo = Foo(prop='foo')
>>> foo.put()
>>> Foo.is_unique(prop='foo')
>>> Foo.is_unique(prop='bar')

Unlike the UniqueByAncestryMixin, the is_unique() method takes a set of keyword arguments matching the property-value pairs. Only the arguments whose names match the properties listed in the unique_properties list will be used. The method performs a count() query internally to test for existence of any entities matching the specified properties, and returns True if there are none.

The current implementation allows a bit more flexibility than useful. There are no checks to catch the situations where properties listed in unique_properties list are proper properties (you will get an AttributeError when you call is_unique with wrong properties listed), and you are not required to test all listed properties either when calling is_unique(). It’s up to the developer to make sure uniqueness tests are successful.

Also note that the query performed in is_unique() method is not an ancestor query, so this method cannot be used inside transactions.


OwnershipMixin is used to assign owners to entities. The ownership is established through a KeyProperty named owner. The kind of the owner entity should be called ‘User’, and owner is required.

The mixin provides two methods. One is the assign_owner() method, which takes either an owner entity or its key and assigns the key to the owner property. The other method is is_owner() which takes an owner entity or its key and tests if the entity is owned by the entity.

The mixin also provides a classmethod, get_by_owner() which takes either an owner entity or its key and returns a query object filtered by owner.


This mixin provides methods for validating model instances on put() or manually. The API for this mixin is still being worked out, so consider it strictly experimental.

Validation uses FormEncode under the hood, so you will need to be(come) familiar with its API.

The model should have a validation schema, which is a simple dictionary mapping property names to validators. At the moment, we are not using the FormEncode’s Schema class, but expect the dictionary schema to be replaced with FormEncode schema in future.

Here is a simple example with an email field:

>>> from google.appengine.ext import ndb
>>> from formencode.validators import Email
>>> from ndb_utils.models import ValidatingMixin
>>> class Foo(ValidatingMixin, ndb.Model):
>>>     validate_schema = {'prop': Email()}
>>>     prop = ndb.StringProperty()
>>> f1 = Foo(prop='invalid_email')
>>> f1.put()
Traceback (most recent call last):
ValidationError: ...
>>> f2 = Foo(prop='')
>>> f1.put()

The ValidationError exception can be accessed as a property on the model:

>>> try:
...     f1.put()
... except Foo.ValidationError:
...     print 'Not a valid email'

Internally, when put() is called, the clean() instance method is called in the _pre_put() hook. This method goes over all keys in the schema, and calls the validator’s to_python() method on the value of the property. If the validator raises formencode.Invalid exception, it remembers the error and continues. When all validation schema keys are processed, it raises the ValidationError exception if there had been any errors.

Repeated properties are currently not supported. This is planned for future releases. Meanwhile, you can create a custom validator to validate repeated properties.

If you prefer to always validate manually, you can set the validate_on_put class property to False and call the clean() method manually.

The clean() method returns cleaned data, instead of assigning them to properties, so you will need to call populate() on the instance to assign the new values. For instance:

>>> try:
...     f1.populate(**f1.clean())
... except Foo.ValidationError:
...     print 'Not a valid email'

Property classes

The NDB utils provide a few property classes that provide features not available in the NDB API by default.

This is a StringProperty that validates strings that shoudl be in slug format (only containing letters A to Z, digits, underscores and dashes).

This is StringProperty that validates email addresses.

This property stores decimals (python’s decimal.Decimal type). The values are internally stored as integers and querying with comparison operators such as >= or < is supported. The floating point precision can be specified, which is used when converting between decimals and integers. This value is specified using float_prec argument and is 2 by default.

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