Given a lat/long how to find the nearest node in a route calculated by pgr_dijkstra?

I’m creating a table route_<route_id> for the route given by pgr_dijkstra, like this. In my application, I have a websocket with a mobile client that it’s sending it’s current position every x seconds. How can I find the nearest node from the table route_<route_id> and return a subset of the route, from the current position to the destination?

The SQL I’m using to generate the route table is:

CREATE TABLE route_route_id as SELECT seq, cost, 
             length::double precision, geom, 
             ST_AsGeoJSON(ST_LineMerge(geom))::json->'coordinates' as coordinates, 
             ST_AsGeoJSON(geom) 
             FROM pgr_dijkstra('
                SELECT gid AS id, source::integer, 
                       target::integer, length AS cost 
                           FROM "ways",
                               convert_to_lon_lat(lat, long, '"ways_vertices_pgr"'), 
                               convert_to_lon_lat(lat, long, '"ways_vertices_pgr"'), 
                               false, false) a LEFT JOIN ways b ON (a.id2 = b.gid);