get_by_id() will not return model instance

Question

I have a Model called Version that looks like this:

from google.appengine.ext import db
import piece

class Version(db.Model):
    "A particular version of a piece of writing."

    parent_piece = db.ReferenceProperty(piece.Piece, collection_name='versions')
    "The Piece to which this version belongs."

    note = db.TextProperty()
    "A note from the Author about this version."

    content = db.TextProperty()
    "The actual content of this version of the Piece."

    published_on = db.DateProperty(auto_now_add=True)
    "The date on which the version was published."

I would like to access instances of Version via their IDs, using Version.get_by_id(), but this call always returns None. I can see in the Datastore Viewer that they have ID values, and in the debugger, I can query for them but not use them:

>>> for each_ver in version.Version.all():
...  print each_ver.key().id()
... 
34
35
36
31
32
>>> a = version.Version.get_by_id(34)
>>> type(a)
<type 'NoneType'>

I see that there are plenty of questions here where people are able to use get_by_id() effectively just as I wish, and they do not see the results that I am seeing.

Could the problem be that each Version instance is a child in an Entity Group rather than a root of an Entity Group? Each Version lives in an Entity Group that looks like Member->Piece->Version. If that is the problem, is there a way that I can refer to Version entity without using its entire key? If that is not the problem, can anyone tell me what I can do to make get_by_id() work as expected?

Answer 1

Could the problem be that each Version instance is a child in an Entity Group rather than a root of an Entity Group?

Yes. An entity's key includes the keys of any parent entities.

If that is the problem, is there a way that I can refer to Version entity without using its entire key?

No. An entity is uniquely identified only by its entire key, which includes the keys of all the parent entities. If you know the kinds of its parent entities, though, you can use db.Key.from_path to construct the key from the chain of IDs or key names.