Please list the models of all the devices with an event under the location coordinates (121, 31).
models of the devices refers to device_model; location coordinates (121, 31) refers to longitude = 121 AND latitude = 31;
SELECT T2.device_model FROM events AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.longitude = 121 AND T1.latitude = 31