First you need to change your model. In relation database you would have:
db.define_table("articles", db.Field("title"), ...)
db.define_table("comments", db.Field("article_id", db.articles),
db.Field("author"), ...)
But with datastore you want to do the reverse, keep the comment ids in the
article table:
db.define_table("comments", db.Field("author"), ...)
db.define_table("articles", db.Field("title"), db.Field("comments",
"list:reference comments"), ...)
or maybe even simply:
db.define_table("articles", db.Field("title"), db.Field("comments",
"list:integer"), ...)
Then you read all the articles as usual:
articles = db().select(db.articles.ALL)
You go over all articles and store keys for all comments:
from google.appengine.ext import db as gdb
keys = []
for a in articles:
keys.extend([gdb.Key.from_path("comments", id) for id in a.comments])
And finally you fetch all comments in a single query:
comments = gdb.get(keys)
You will get the comments in a fixed order so it shouldn't be difficult to
map them to articles in the view.
Simply for the first article where len(articles[0].comments) = 3, it would
be comments[:3],
for the second article where len(articles[1].comments) = 5, it would be
comments[3:8].
A dictionary with article ids as you proposed would also work.
