Hi louvail2:
Some things for you to consider.
- If you want to update an existing record in a data collection you need the _id field for that record.
check this out: wix-data - Velo API Reference - Wix.com
So what you may want to do is read the record from the data collection first (unless you know the _id) then
update the record you retrieve with the ‘Mandat’ value.
Now for folks to help you solve your problem if you share the schema for each of your data collections and how they are connected that will help. Also a screen shot and the URL for the page you are trying to get to work will be good along with a description of what you are trying to do.
My brief assessment is that you are not using the promises for the data api correctly (in addition to my comment about the _id above). But cant say for sure until there is more data to go on.