Flask Python Dynamic WTForms Fields Javascript Ajax

I require a userform using Flask_WTForms that has some static select and string fields but then creates new selectfields dynamically depending on which value is selected in a particular selectfield. E.g: existing form with Selectfield, select from 4 options. Use selection to do a DB lookup and return 4 strings, for each of these strings do another db lookup and return 3 option values, create a new selectfield with these 3 choices. When submitting the entire form capture the new selectfield labels and values. Also, when selecting a different value from the static selectfield, do the same but if there are now less fields remove the extras. This requires the use of JS/Ajax which I am extremely unfamilar with. The database component is not require in this project. I have made a decent start but it doesn't quite work, only when submitting the form (sending a POST) will the new fields render. Happy to provide this but I don't think it's the right way to go. I hope that makes sense, happy to clairfy if required.