Can you help me understand the nbd Key Class Docum

Question

I am trying to wrap my head 'round gae datastore, but I do not fully understand the documentation for the Key Class / or maybe it is ancestor relationships in general I do not grasp. I think what I want is multiple ancestors.

Example: Say I wanted to model our school's annual sponsored run for charity; school kids run rounds around the track and their relatives (=sponsors) donate to charity for each round completed.

In my mind, I would create the following kinds:

1. Profile (can be both runner and sponsor)
2. Run (defines who (cf. profile) runs for what charity, rounds actually completed)
3. Sponsorship (defines who (cf. profile) donates how much for what run, whether the donation has been made)

I've learned that datastore is a nosql, non-relational database, but haven't fully grasped it. So my questions are:

a. Is creating an entity for "Sponsorship" even the best way in datastore? I could also model it as a has-a relationship (every run has sponsors) - but since I also want to track the amount sponsored, whether sponsor paid up and maybe more this seems inappropriate

b. I'd like to easily query all sponsorhips made by a single person and also all sponsorships belonging to a certain run. So, I feel, this would be appropriate:

Profile --is ancestor of--> Run
Profile --is ancestor of--> Sponsorship
Run --is ancestor of--> Sponsorship

Is that sensible? I can see a constructor for a Key that takes several kinds in ancestor order as arguments. Was that designed for this case? "Run" and "profile" would be at the same "level" (i.e. mum&dad ancestors not father&grandfather) - what would that constructor look like in python?

Answer 1

The primary way of establishing relationships between entities is via the key properties in the entity model. Normally no ancestry is needed.

For example:

class Profile(ndb.Model):
    name = ndb.StringProperty()

class Run(ndb.Model):
    runner = ndb.KeyProperty(kind='Profile')
    rounds = ndb.IntegerProperty()
    sponsorship = ndb.KeyProperty(kind='Sponsorship')

class Sponsorship(ndb.Model):
    run = ndb.KeyProperty(kind='Run')
    donor = ndb.KeyProperty(kind='Profile')
    done = ndb.BooleanProperty()

The ancestry just places entities inside the same entity group (which can be quite limiting!) while enforcing additional relationships on top of the ones already established by the model. See Transactions and entity groups and maybe Contention problems in Google App Engine.