https://docs.cloud.google.com/appengine/docs/standard/python3/reference/services/bundled/google/appengine/datastore/datastore_query/CorrelationFilter
google.appengine.datastore.datastore_query.CorrelationFilter | App Engine standard environment |...
A filter that isolates correlated values and applies a sub-filter on them.
app enginegoogleappenginedatastorequery