Question

I am somewhat new to the google app engine models system(and all database models for that matter), and I am trying to figure out how make a model have a collection of model instances as one of the properties. Some how I feel that there has to be a better way to model the data I have.

So here is the example. I’m writing a journal webapp. I want to allow users to have multible journals as defined by the model Journals

#Represents a collection of journals
class Journals(db.Model):
    user = db.UserProperty()
    journals = db.ListProperty(int)  #there has to be a better way of keeping track of journals then by a list of their id

In the property journals I keep a list of the id’s of the Journal model(shown below).

class Journal(db.Model):
    user = db.UserProperty()
    name = db.StringProperty()
    date_created = db.DateTimeProperty(auto_now_add=True)
    last_modified = db.DateTimeProperty(auto_now=True)
    journal_item = db.ListProperty(int)

In the Journal model I store a list of id’s of JournalItem instances as a list of id’s

class JournalItem(db.Model):
    user = db.UserProperty()
    name = db.StringProperty()
    date_created = db.DateTimeProperty(auto_now_add=True)
    last_modified = db.DateTimeProperty(auto_now=True)
    content = db.StringProperty(multiline=True)

So I guess the question really is how should I model data with this structure(I omitted some properties to simplify it)(shown in json).

"Journals": {
    "Journal": {
        "user": "GAE user object",
        "name": "journal 1",
        "JournalItems": {
            "JournalItem": {
                "name": "entry 1",
                "content": "some example content"
            },
            "JournalItem": {
                "name": "entry 2",
                "content": "some example content"
            },
            "JournalItem": {
                "name": "entry n",
                "content": "some example content"
            }
        }
    }
    "Journal": {
        "user": "GAE user object",
        "name": "journal 2",
        "JournalItems": {
            "JournalItem": {
                "name": "entry 1",
                "content": "some example content"
            },
            "JournalItem": {
                "name": "entry 2",
                "content": "some example content"
            },
            "JournalItem": {
                "name": "entry n",
                "content": "some example content"
            }
        }
    }
}

I hope that wasn’t to long-winded to be annoying. Any help on the matter would be greatly appreciated.

Answer 1

That’s pretty much the way to do it, except I would use a ListProperty(db.Key) instead of int. Then you can do db.get(userjournals.journals) to get all the user’s journals once you have the Journals instance belonging to that user (note that to avoid confusion, you should probably call that something like UserJournals).

Remember that GAE is a non-relational datastore, and sometimes you have to think a bit differently from how you would using SQL.