Using ancestors or reference properties in Google App Engine?

Question

Currently, a lot of my code makes extensive use of ancestors to put and fetch objects. However, I'm looking to change some stuff around.

I initially thought that ancestors helped make querying faster if you knew who the ancestor of the entity you're looking for was. But I think it turns out that ancestors are mostly useful for transaction support. I don't make use of transactions, so I'm wondering if ancestors are more of a burden on the system here than a help.

What I have is a User entity, and a lot of other entities such as say Comments, Tags, Friends. A User can create many Comments, Tags, and Friends, and so whenever a user does so, I set the ancestor for all these newly created objects as the User.

So when I create a Comment, I set the ancestor as the user:

comment = Comment(aUser, key_name = commentId)

Now the only reason I'm doing this is strictly for querying purposes. I thought it would be faster when I wanted to get all comments by a certain user to just get all comments with a common ancestor rather than querying for all comments where authorEmail = userEmail.

So when I want to get all comments by a certain user, I do:

commentQuery = db.GqlQuery('SELECT * FROM Comment WHERE ANCESTOR IS :1', userKey)

So my question is, is this a good use of ancestors? Should each Comment instead have a ReferenceProperty that references the User object that created the comment, and filter by that?

(Also, my thinking was that using ancestors instead of an indexed ReferenceProperty would save on write costs. Am I mistaken here?)

Answer 1

You are right about the writing cost, an ancestor is part of the key which comes "free". using a reference property will increase your writing cost if the reference property is indexed.
Since you query on that reference property if will need to be indexed.

Ancestor is not only important for transactions, in the HRD (the default datastore implementation) if you don't create each comment with the same ancestor, the quires will not be strongly consistent.

-- Adding Nick's comment ---
Every entity with the same parent will be in the same entity group, and writes to entity groups are serialized, so using ancestors here will slow things down iff you're writing multiple entities concurrently. Since all the entities in a group are 'owned' by the user that forms the root of the group in your instance, though, this shouldn't be a problem - and in fact, what you're doing is actually a recommended design pattern.