GTFS realtime API query does not return some feeds

[jcpitre]

Describe the bug

Some GTFS RT feeds are not listed when queried.
It results in integration tests periodically failing: See here

If I do:

curl -X 'GET' \
  'https://api.mobilitydatabase.org/v1/feeds?provider=AMB%20Mobilitat' \
  -H 'accept: application/json' \
  -H 'Authorization: Bearer ...'

i.e. the /v1/feeds endpoint with provider AMB Mobilitat I get 2 feeds (mdb-2368 and mdb-2369) both with "data_type": "gtfs_rt". It's also the 2 feeds if I search with mobilitydatabase.org. So far so good.
But if I do the same with the /v1/gtfs_rt_feeds, I get nothing. i.e.:

curl -X 'GET' \
  'https://api.mobilitydatabase.org/v1/gtfs_rt_feeds?provider=AMB%20Mobilitat' \
  -H 'accept: application/json' \
  -H 'Authorization: Bearer ...'

returns an empty list.

Steps/Code to Reproduce

See above

Expected Results

A list of 2 GTFS realtime feeds

Actual Results

An empty list.

Screenshots

No response

Files used

No response

Additional notes

No response

[jcpitre]

I tracked the problem to the fact that the two GTFS RT feeds that should be listed but are not (mdb-2368 and mdb-2369) is because they have no location (ie no entry in the locationfeed table).

The offending code is in db_utils.py where we have a join with the Location table, but the feeds are not present in the locationfeed table.

One suggestion is to add isouter=True to the join like this:

    subquery = gtfs_rt_feed_filter.filter(
        select(Gtfsrealtimefeed.id)
        .join(Location, Gtfsrealtimefeed.locations, isouter=True)
        .join(Entitytype, Gtfsrealtimefeed.entitytypes)
    ).subquery()

[jcpitre]

Implemented the suggested fix in #918
If it happens, the returned list of locations for the feed is enpty.