Change IntegerProperty to FloatProperty of existing AppEngine DataStore

Question

I built an appengine application (python) which need to convert existing datastore entities in integer value (100) to float value (100.00) for currency conversion issue. How's the right way doing this? Since my query returning error when i just change property type in my model.

Old Model:

class Learn(search.SearchableModel):
    pid = db.ReferenceProperty(Product, collection_name='picks')
    title = db.StringProperty()
    description = db.TextProperty()
    order = db.IntegerProperty()
    cost = db.IntegerProperty(default=0)
    cost1 = db.IntegerProperty(default=0)

New Model:

class Learn(search.SearchableModel):
    pid = db.ReferenceProperty(Product, collection_name='picks')
    title = db.StringProperty()
    description = db.TextProperty()
    order = db.IntegerProperty()
    cost = db.FloatProperty(default=0.000)
    cost1 = db.FloatProperty(default=0.000)

I need a proper way to alter this datastore property type without changing (delete old and add new) existing data. Since it's key used in many others table/model.

Thanks.

Answer 1

The easiest way to do this is to change the model to inherit from db.Expando, and delete the integer properties from the definion. Then, load each instance and do "instance.foo = float(instance.foo)" on each, before saving them back to the datastore - you'll probably want to use the mapreduce API for this. Finally, make the model extend db.Model again, and add the FloatProperties back.

You really, really don't want to use a float for currency, though: floats are susceptible to rounding errors, which means you can lose (or gain!) money. Instead, use an IntegerProperty that counts the number of cents.

Answer 2

Here is exampe for Nick Johnson's answer:

Before:

class Person(db.Model):
    name = db.StringProperty()
    age = db.StringProperty() #this will go to int

After

class Person(db.Expando):
    pass

for person in Person.all():
    person.age = int(person.age)
    person.put()

Very after:

class Person(db.Model):
    name = db.StringProperty()
    age = db.IntegerProperty() 

Answer 3

Maybe a good way is to temporary create a new model:

class LearnTemp(search.SearchableModel):
    pid = db.ReferenceProperty(Product, collection_name='picks')
    title = db.StringProperty()
    description = db.TextProperty()
    order = db.IntegerProperty()
    order = db.IntegerProperty()
    cost = db.FloatProperty(default=0.000)
    cost1 = db.FloatProperty(default=0.000)

Then write some script, task or view which convert instances from old model to temporary model, converting the integer values to float. Make sure copying the id's and key's as well if somehow possible.

After change your main model and copy all entries from the temporary one to it. Then remove the temporary model.

This is most likely not the optimal way and requires some manual migration, though without South and on app engine I don't really see a good way to do it.

Answer 4

From the Edit Entity page in the data store admin interface:

Enter information for the entity below. If you'd like to change a property's type, set it to Null, save the entity, edit the entity again, and change the type