constructing display_name field

[xinyuluo]

I noticed that in Nominatim output, there's a display_name field which is usually the "address" of the map object of interest. Given that sometimes location properties (e.g. county, city, locality, street, etc.) are optional arguments, they exist for some object and not others, and we don't know when they exist and when not.
Can someone point to me the code responsible for generating display_name field in Nominatim? I'm interested to see how it handles optional arguments and arrange different regional names into a hierarchical order.

[mtmail]

The magic happens in SQL in lib-sql/functions/address_lookup.sql. Look for get_address_by_language. Adding &debug=1 to the request URL will show how it's called https://nominatim.openstreetmap.org/search.php?q=Schwanthalerstra%C3%9Fe+53%2C+munich&format=jsonv2&debug=1&accept-language=de

Example:

-- https://www.openstreetmap.org/node/2469452675
select place_id from placex where osm_type='N' and osm_id=2469452675;
 22706553

select get_address_by_language(22706553, -1, ARRAY['name:en']);
53, Schwanthalerstraße, Klinikviertel, Ludwigsvorstadt-Kliniken, 2, Munich, Bavaria, 80336, Germany

select get_address_by_language(22706553, -1, ARRAY['name:de']);
53, Schwanthalerstraße, Klinikviertel, Ludwigsvorstadt-Kliniken, Ludwigsvorstadt-Isarvorstadt, München, Bayern, 80336, Deutschland

select get_address_by_language(22706553, -1, ARRAY['name:es']);
53, Schwanthalerstraße, Klinikviertel, Ludwigsvorstadt-Kliniken, 2, Múnich, Baviera, 80336, Alemania

select get_addressdata(22706553, -1);
 (113712314,W,37665695,"""name""=>""Schwanthalerstraße""",highway,secondary,,15,t,t,26,0)
 (210006,N,54129121,"""name""=>""Klinikviertel""",place,quarter,,15,f,t,22,0.004454800373751082)
 (13784268,N,1374687395,"""name""=>""Ludwigsvorstadt"", ""name:de""=>""Ludwigsvorstadt"", ... (290498591,R,65911,"""name""=>""Ludwigsvorstadt-Kliniken""",boundary,administrative,,10...
 (290500507,R,54392,"""ref""=>""2"", ""name""=>""Ludwigsvorstadt-Isarvorstadt"", ""name:de""=>...
 (290495803,R,62428,"""name""=>""München"", ""name:ar""=>""ميونخ"", ""name:az""=>""Münhen"", ... (290758640,R,2145268,"""ref""=>""BY"", ""name""=>""Bayern"", ""name:af""=>""Beiere"", ""name:a...
(290635030,R,1100784,"""ref""=>""80336""",boundary,postal_code,,15,t,f,5,0.007132745808835589)
 (,,,"""name""=>""Deutschland"", ""name:ab""=>""Алмантәыла"", ""name:af""=>""Duitsland"", ""name:ak""=>""Germany"",...```

Have a look at https://github.com/OpenCageData/address-formatting/ for an alternative approach to generate an address string from a list of address parts.