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Editorial Team
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Asked: 2026-06-13T05:53:50+00:00

I want to get an entity key knowing entity ID and an ancestor. ID

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I want to get an entity key knowing entity ID and an ancestor.
ID is unique within entity group defined by the ancestor.
It seems to me that it’s not possible using ndb interface. As I understand datastore it may be caused by the fact that this operation requires full index scan to perform.
The workaround I used is to create a computed property in the model, which will contain the id part of the key. I’m able now to do an ancestor query and get the key

class SomeModel(ndb.Model):
    ID = ndb.ComputedProperty( lambda self: self.key.id() )

    @classmethod
    def id_to_key(cls, identifier, ancestor):
        return cls.query(cls.ID == identifier,
                         ancestor = ancestor.key ).get( keys_only = True)

It seems to work, but are there any better solutions to this problem?

Update
It seems that for datastore the natural solution is to use full paths instead of identifiers. Initially I thought it’d be too burdensome. After reading dragonx answer I redesigned my application. To my suprise everything looks much simpler now. Additional benefits are that my entities will use less space and I won’t need additional indexes.

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1 Answer

  1. Editorial Team

    I ran into this problem too. I think you do have the solution.

    The better solution would be to stop using IDs to reference entities, and store either the actual key or a full path.

    Internally, I use keys instead of IDs.

    On my rest API, I used to do http://url/kind/id (where id looked like “123”) to fetch an entity. I modified that to provide the complete ancestor path to the entity: http://url/kind/ancestor-ancestor-id (789-456-123), I’d then parse that string, generate a key, and then get by key.

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