Question

Using AppEngine datastore, but this might be agnostic, no idea.

Assume a database entity called Comment. Each Comment belongs to a User. Every Comment has a date property, pretty standard so far.

I want something that will let me: specify a User and get back a dictionary-ish (coming from a Python background, pardon. Hash table, map, however it should be called in this context) data structure where:

• keys: every date appearing in the User‘s comment
• values: Comments that were made on date.

I guess I could just iterate over a range of dates an build a map like this myself, but I seriously doubt I need to “invent” my own solution here.

Is there a way/tool/technique to do this?

Answer 1

Datastore supports both references and list properties. This let’s you build one-to-many relationships in two ways:

1. Parent (User) has a list property containing keys of Child entities (Comment).

2. Child has a key property pointing to Parent.

Since you need to limit Comments by date, you’d best go with option two. Then you could query Comments which have date=somedate (or date range) and where user=someuserkey.

There is no native grouping functionality in Datastore, so to also “group” by date, you can add a sort on date to the query. Than when you iterate over the result, when the date changes you can use/store it as a grouping key.

Update

Designing no-sql databases should be access-oriented (versus datamodel oriented in sql): for often-used operations you should be getting data out as cheaply (= as few operations) as possible.

So, as a rule of thumb you should, in one operation, only get data that is needed at that moment (= shown on that page to user). I’m not sure about your app’s design, but I doubt you need all user’s full comments (with text and everything) at one time.