Question

GAE db.Model cannot store a list of tuples. Thus I need to save the list as a json.dumps string and json.loads the string when I pull the list back out of the entity. I would like to automate this. Here is my attempt:

class example(db.Model):
    someAttr = db.StringListProperty()
    A = db.StringProperty()
    B = db.StringProperty()
    C = db.StringProperty()

    def __setattr__(self, name, value):
        #convert json to string for storage
        if name in ("A", "B", "C"):
            value = json.dumps(value)
        #call default set method
        return super(Quote, self).__setattr__(name, value)

    def __getattribute__(self, name):
        #convert string to json for retrieval
        if name in ("A", "B", "C"):
            val = super(Quote, self).__getattribute__(name)
            return json.loads(val)
        else: 
            # default get behavior
            return super(Quote, self).__getattribute__(name)

When I try to set the A,B,C attributes of the entity I get:

BadValueError: Property C must be a str or unicode instance, not a list

Answer 1

If you don’t need to store very large JSONs, I’d suggest you check out NDB. It has built-in JsonProperty: https://developers.google.com/appengine/docs/python/ndb/properties#types.

I also came across an interesting post in App Engine Googl Group that talks about a more efficient way of storing large JSONs: https://groups.google.com/forum/#!msg/google-appengine/WPfAvHDGNjQ/XfakEMm1qzoJ