HAS ANCESTOR and HAS DESCENDANT clauses in google cloud datastore

问题

I'm studying the Google Cloud Datastore GQL grammar - specifically the HAS ANCESTOR and HAS DESCENDANT comparison operators.

Giving the following Person entities:

• Amy
• Fred, parent = Amy
• Laura, parent = Amy
• Paul
• Agnes ...

Would the GQL queries below produce the same output?

SELECT * FROM Person WHERE key_name='Fred' HAS ANCESTOR KEY('Person', 'Amy')

SELECT * FROM Person WHERE KEY('Person', 'Amy') HAS DESCENDANT key_name='Fred'

If so, I don't understand the existence of HAS DESCENDANT clause.

Thanks in advance!

回答1:

These two GQL queries should produce identical results:

SELECT * FROM Person WHERE __key__ HAS ANCESTOR KEY('Person', 'Amy')
SELECT * FROM Person WHERE KEY('Person', 'Amy') HAS DESCENDANT __key__

来源：https://stackoverflow.com/questions/20553266/has-ancestor-and-has-descendant-clauses-in-google-cloud-datastore