I am trying to load an entity by key using the id it was assigned by the datastore but I don't see any api method to do this (using NDB). I thought I would be able to make a Key from an integer id and use key.get() to load the entity, but I don't see a way to make a key from just an id. I suspect I am missing something obvious here. How should I load an entity where I only know the id of it?
Another way: ndb.Key(YourModel, id).get().
YourModel.get_by_id() gets a model instance by id.
here the docs:
https://developers.google.com/appengine/docs/python/ndb/modelclass#Model_get_by_id
don't think you can't get an entity by id without knowing the kind because instances of different Model classes can have the same id/key_name
Models in NDB don't define their key type as part of the model. This is nifty in that you can have one given model type that is accessible through multiple different kinds of keys and parents, which makes them more flexible. But it's somewhat problematic because it isn't always clear what the key represents or where it comes from.
So in cases where there's only ever one kind of key for a given model (which is almost every model), I like to create a class method for generating that key, which adds a bit of semantic clarity:
class Book(ndb.Model):
title = ndb.StringProperty()
pages = ndb.IntegerProperty()
#classmethod
def make_key(cls, isbn):
return ndb.Key(cls, isbn)
b = Book.make_key('1234-5678').get()
Sure the added code is not strictly necessary, but it adds clarity and makes my models more long-term maintainable.
You can parse the id to key string:
key = ndb.Key(YourModel, id).urlsafe().
and then:
result = YourModel.query(YourModel.key== key).get().
Related
Just getting started with Wagtail. To support interoperability with a legacy system, I'd like to have the id/pk of my Page objects be UUIDs instead of Integers. I tried just adding a id = models.UUIDField(primary_key=True, editable=False, default=uuid.uuid4) to my class that inherits from Page but I get an error Local field u'id' clashes with field of similar name from base class 'Page'
Is there a simple way to make id be a UUID? Or, do I just need to call it something besides id?
There won't be a simple way to do this, unfortunately - the assumption that IDs are numeric is baked in to the database schema, URL routes, rich text data representation and various other places in Wagtail's design.
Would it be an option to add your UUID column as a new field on your model (named something like legacy_id), and look up on that whenever you need to interoperate with the legacy system - but otherwise leave the primary key as numeric?
I'm quite new with Django, and so far I have a pretty basic setup where I attach an extra model to the default User model from (django.contrib.auth.models). In my accounts.models.py I have something in the line of:
class UserExtension(models.Model):
user = models.OneToOneField(User, primary_key=True)
# more code
I also have another model which needs to be specified an owner. My question is: Which is the better (more django-ish, more readable, more efficient, more flexible) way to signify the owner:
class Owned(models.Model):
# code
owner = models.ForeignKey(User)
# more code
or:
class Owned(models.Model)
# code
owner = models.ForeignKey(UserExtension)
# more code
I'll really appreciate if you mention pros and cons of those approaches.
I'd recommend the first option. The user model is the nontrivial model in the sense that an owned object should not be able to exist without a user, but could exist without a UserExtension.
Also consider that in Django 1.5 you are able to create custom user model, eliminating the need for the UserExtension class. See the documentation for more information.
Consider using UserProfile for any per user add-on information. Check out this blog to see how to do it. Then you can be assured that you are creating UserProfile object every time you create the User.
Now whether you FK on User or UserProfile depends logically on what you are doing within Owned. If Owned works with User's data/field, FK on User; if it works with UserProfile's data, FK on UserProfile.
I am try to persist an instance (entity) that contains a predefined instances(entities), with JPA and GAE, the relation is oneToOne, as the following:
#Entity class Address{ ... #OneToOne(cascade = CascadeType.ALL)private City city;}
#Entity class City{...}
I created city instance(entity) and persist it, works good, when trying to create Address instance that contains the created city instance(since from UI, it selected from dropdown box), I got the following exception:
javax.persistence.PersistenceException: Detected attempt to establish Address(no-id-yet) as the parent of City(20) but the entity identified by City(20) has already been persisted without a parent. A parent cannot be established or changed once an object has been persisted.
Is there any annotation(s) should be used in order to make this happen?
Thanks in advance
The problem is how DataStore stores keys. Your type of relation is Parent/Child, so for City you will have Adress as a Parent. This mean that you will have composite key with parent as an Address. Your City was already persists so you will not be able to change key (keys can not be changes dynamically after creation. They are immutable).
I do not think that OneToOne reference is suitable for you. Why do you need a separate object? If you really need it - do not try to keep reference integrity. It is very expensive in NoSQL and could cause a lot of problems.
I want to ensure an object is unique, and to throw an error when a user tries to save it (e.g. via the admin) if not? By unique, I mean that some of the object's attributes might hold the same values as those of other objects, but they can't ALL be identical to another object's values.
If I'm not mistaken, I can do this like so:
class Animal(models.Model):
common_name = models.CharField(max_length=150)
latin_name = models.CharField(max_length=150)
class Meta:
unique_together = ("common_name", "latin_name")
But then each time I refactor the model (e.g. to add a new field, or to change the name of an existing field), I also have to edit the list of fields in the parenthesis assigned to unique_together. With a simple model, that's OK, but with a substantial one, it becomes a real hassle during refactoring.
How can I avoid having to repeat typing out the list of field names in the unique_together parenthesis? Is there some way to pass the list of the model's fields to a variable and to assign that variable to unique_together instead?
Refactoring models is a rather expensive thing to do:
You will need to change all code using your models since field names correspond to object properties
You will have to change your database manually since Django cannot do this for you (at least the version I used the last time when I worked with Django couldn't)
Therefore I think updating the list of unique field names in the model meta class is the least issue you should worry about.
EDIT: If you really want to do this and all of your fields must be "unique together", then the guy at freenode is right and you'll have to write a custom metaclass. This is quite complicated and errorprone, plus it might render your code incompatible to future releases of Django.
Django's ORM "magic" is controlled by the metaclass ModelBase (django.db.models.base.ModelBase) of the generic base class Model. This class is responsible to take your class definition with all fields and Meta information and construct the class you will be using in your code later.
Here is a recipe on how you could achieve your goal:
Subclass ModelBase to use your own metaclass.
Override the method __new__(cls, name, bases, dict)
Inspect dict to gather the Meta member (dict["Meta"]) as well as all field members
Set meta.unique_together based on the names of the fields you gathered.
Call the super implementation (ModelBase.__new__)
Use the custom metaclass for all your unique models using the magic member __metaclass__ = MyMetaclass (or derive an abstract base class extending Model and overriding the metaclass)
I would like to create a Group model in Google App Engine and then have an attribute where I can create a list of UserReferences. The documentation said:
"A property can have multiple values, represented in the datastore API as a Python list. The list can contain values of any of the value types supported by the datastore."
Would I implement this by creating:
class Group(db.Model):
group_list = db.ListProperty(users.User)
Or might I be better served by simply listing the user entity keys?
http://code.google.com/appengine/docs/python/datastore/entitiesandmodels.html
keys are better placed in ReferenceProperty and their purpose is to create relationships between two kinds.
You can simply create the listproperty and as your list grows keep adding listitems to it.
class Group(db.Model):
group_list = db.ListProperty()
This depends on your use-case. If you already have a User model, to store additional data about your users, then using a db.ListProperty(Key) for User model keys is probably your best option.