Is there some way to perform a datastore.Get by providing not the full path of the key, but, say, the root parent of an entity?
For instance, say there's a SubFolder entity, under a Folder entity which itself is under a User entity. This doesn't work:
subFolderKey := datastore.NewKey(c, "SubFolder", "mySubfolder", 0, userKey)
datastore.Get(c, subFolderKey, subFolder)
even though mySubfolder is a grandchild of userKey.
Is there some way to do this without specifying full parent path?
No, that is not possible. The ancestor path is the full key of your entity so if you leave something out, you will have an incomplete key.
See also this SO answer.