Use a db.StringProperty() as unique identifier in Google App Engine

Question

Hi,

I just have a hunch about this. But if feels like I'm doing it the wrong way. What I want to do is to have a db.StringProperty() as a unique identifier. I have a simple db.Model, with property name and file. If I add another entry with the same "name" as one already in the db.Model I want to update this.

As of know I look it up with:

template = Templates.all().filter('name = ', name)

Check if it's one entry already:

if template.count() > 0:

Then add it or update it. But from what I've read .count() is every expensive in CPU usage.

Is there away to set the "name" property to be unique and the datastore will automatic update it or another better way to do this?

..fredrik

Answer 1

You can just try to get your entity and edit it, and if not found create a new one:

template = Templates.gql('WHERE name = :1', name)
if template is None:
  template = Templates()

# do your thing to set the entity's properties

template.put()

That way it will insert a new entry when it wasn't found, and if it was found it will update the existing entry with the changes you made (see documentation here).

Answer 2

You can't make a property unique in the App Engine datastore. What you can do instead is to specify a key name for your model, which is guaranteed to be unique - see the docs for details.

Answer 3

I agree with Nick. But, if you do ever want to check for model/entity existence based on a property, the get() method is handy:

template = Templates.all().filter('name = ', name).get()
if template is None:
  # doesn't exist
else:
  # exists

Answer 4

I was having the same problem and came up with the following answer as the simplest one :

class Car(db.Model):
  name = db.StringProperty(required=True)

  def __init__(self,*args, **kwargs):
      super(Car, self).__init__(*args, **kwargs)
      loadingAnExistingCar = ("key" in kwargs.keys() or "key_name" in kwargs.keys())
      if not loadingAnExistingCar:
          self.__makeSureTheCarsNameIsUnique(kwargs['name'])


  def __makeSureTheCarsNameIsUnique(self, name):
      existingCarWithTheSameName = Car.GetByName(name)
      if existingCarWithTheSameName:
          raise UniqueConstraintValidationException("Car should be unique by name")

  @staticmethod
  def GetByName(name):
      return Car.all().filter("name", name).get()

It's important to not that I first check if we are loading an existing entity first.

For the complete solution : http://nicholaslemay.blogspot.com/2010/07/app-engine-unique-constraint.html