Use key_name instead of link_id
Some entities do not have a link_id, but all entities are guaranteed
to have a key_name (or an id).
indexes:
# AUTOGENERATED
# This index.yaml is automatically updated whenever the dev_appserver
# detects that a new type of query is run. If you want to manage the
# index.yaml file manually, remove the above marker line (the line
# saying "# AUTOGENERATED"). If you want to manage some indexes
# manually, move them above the marker line. The index.yaml file is
# automatically uploaded to the admin console when you next deploy
# your application using appcfg.py.