Question

Assume I have a model definition like this:

class Image(db.Model):
    id = db.StringProperty()
    url = db.URLProperty()

Now I want to add some fields to this model to make it look like this:

class Image(db.Model):
    id = db.StringProperty()
    url = db.URLProperty()
    width = db.IntegerProperty()
    height = db.IntegerProperty()

So, this new model will be applied properly to newly added Image entities. But I also want to update already existing entities so that they contained these two new fields and fill them with values. Will an already existing entity get these two fields automatically so when I refer to them, it will give me empty fields or will it cause an error? I suppose I will have to create a helper function that will go through all existing entities and set new fields values, right? So, what should I keep in mind and how to better do this model update? I think it will happen sometimes as the application emerges, so I think it would be useful to have some straightforward flow to do that.

Answer 1

This exact scenario is covered in the GAE docs (articles section):

Updating your model’s schema.

Basically just change the model definition as you’ve done, then perform some operation to supply default values for all your extant entities. There are several ways to do the second part – the article describes one.