如何在App Engine数据存储区（Java）中查询根实体

Question

I'm trying to write a query which will identify and return all root entities (ie entities which have no ancestor). I initially tried calling Query.setAncestor(null) but later found out from the datastore Query docs that this doesn't work:

Passing null as a parameter to Query.setAncestor(String ancestor) does not query for entities without ancestors (this type of query is not currently supported).

So now I'm a bit stuck, given what the Query API contains I can't figure out how one would construct a query which identified all the root entities.

This needs to be app functionality so I'm looking for a solution which works programatically as opposed to some manual intervention wnich requires me to log in to the admin console and click a button :-)

Anyone know how to do this?

Cheers,

Edd

Answer 1

You will need to add a property to your models that you can query on. Perhaps a boolean named is_root or something similar.

Answer 2

As the documentation indicates, there's no way to do this - other than working around it as @Robert suggests. If you tell us what you're trying to achieve - as opposed to how you're trying to achieve it - perhaps we can provide a suggestion on how best to go about it.