Visualizing city object from 3DcityDB using Cesium JS

Hello,
I have a 3D city model in CityJSON format stored in a 3DcityDB. I want to visualize it using Cesium JS, but Cesium JS does not support CityJSON directly. I have set up a Flask API to connect Cesium and the database. How can I retrieve the geometries of objects from the database and visualize them on Cesium?
Thank you for your answers!