Question

I have a question regarding how to model a many-to-many relationship in App Engine:

A Blogentry can have many tags, a tag can apply to many blog entries.

I see a couple of scenarios:

Use a Set of Strings as an attribute on the blog entry.
- This allows me to easily query for an entry using a tag
- This does not allow me to fetch all tags and their weights (how many entries they apply to)
Use an unowned relationship between an Entry and a Tag class (Set of keys for Tags in Entry class and vise versa)
- This allows me to fetch all tags and their weights
- This is much more comples to maintain
- Are Set attributes lazyloaded, or would I fetch the entire graph of object every time? (Fetch an Entry, which fetches a number of Tags, each in turn fetching a number of Entries)
use 1. but maintain data on tags and their weights seperately
- This has synchronisation issues between the Tag data and the tags in the Entries

Any input and pointers would be appreciated. I think this is a quite common scenario but I haven t seen any good solutions yet.

Answer 1

Like many other database management systems, many-to-many relationships are not natively supported in App Engine Datastore, but could be solved through a "junction table". However, since App Engine s query language does not support joins, this will be very painful to use in your application. Google s BigTable architecture in fact discourages this, because distributed joins are not efficient.

So, I suggest going with the "keep it simple stupid" rule; use the simplest thing that works. A list of strings in a "Blogentry" object sounds fairly robust. Even if it s prone to race conditions (people making updates in parallel, overwriting each other s changes), but how many people do you have editing the same blog post anyway?

Answer 2

I decided to go with option 3., to maintain a seperate list of tags with their weights.

This seems to work ok, although the insert/update code is a bit cluttered.

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