I think a multiselect dropdown is not a solution for a one-to-many relation.
A solution I am thinking of is a dataview with the single object (from the ‘one' table).
Inside of it you can list out the possible options (from the many table).
In this list, you can make checkboxes or toggle widgets, to let the user select the options.
On the change event of the toggle/checkbox you can then change and commit the option to the relation of the dataview object.