Custom Postgres Update Function - postgresql

Question

I have a table which contains multiple rows for a user, holding their station_ids. When a station ID is changed from the front end via drop down button, I want to update the station ID in the table. I only want there to ever be one TRUE value for "is_default_station", but a user can have multiple FALSE values. I am using postgres 9.5, and the PG drive for NodeJS.
My table looks like this:
station_id | station_name | user_id | is_default_station
-----------------+---------------------------- +-------------------------+--------------------------
1 | station 1 | 1 | TRUE
2 | station 2 | 1 | FALSE
3 | station 3 | 1 | FALSE
4 | station 4 | 2 | FALSE
5 | station 5 | 2 | FALSE
6 | station 6 | 2 | TRUE
Here is my function:
CREATE OR REPLACE FUNCTION UPDATE_All_STATIONS_FUNC (
userId INTEGER,
stationId INTEGER
)
RETURNS RECORD AS $$
DECLARE
ret RECORD;
BEGIN
--Find all the user stations associated to a user, and set them to false. Then, update one to TRUE
UPDATE user_stations SET (is_default_station) = (FALSE) WHERE station_id = ALL (SELECT station_id FROM user_stations WHERE user_id =$1 AND is_default_station = TRUE);
UPDATE user_stations SET (is_default_station) = (TRUE) WHERE station_id =$2 AND user_id = $1 RETURNING user_id, station_id INTO ret;
RETURN ret;
END;
$$ LANGUAGE plpgsql;
I am accessing the function like so:
SELECT user_id, station_id FROM update_all_stations_func($1, $2) AS (user_id INTEGER, station_id INTEGER)
The function is not updating anything on the DB, and returning null values for user_id and station_id like so rows: [ { user_id: null, dashboard_id: null } ].
I am guessing that the initial update query with the nested SELECT is not finding anything inside the function, but if I use the first query alone to update, I find results and it updates as expected. What am I missing?

Answer 1

I simplified the first update statement, and the following works as expected:
CREATE OR REPLACE FUNCTION UPDATE_All_STATIONS_FUNC (
userId INTEGER,
stationId INTEGER
)
RETURNS RECORD AS $$
DECLARE
ret RECORD;
BEGIN
--Find all the user stations associated to a user, and set them to false. Then, update one to TRUE
UPDATE user_stations SET (is_default_station) = (FALSE) WHERE user_id = $1 AND station_id <> $2;
UPDATE user_stations SET (is_default_station) = (TRUE) WHERE user_id = $1 AND station_id = $2 RETURNING user_id, station_id INTO ret;
RETURN ret;
END;
$$ LANGUAGE plpgsql;