Unable change _assign field

i have two tables asset and project,
project have a users and asset is assigned to project
where i create asset with project all users which are present in that project should auto assigned to asset (this part is completed)
now if admin remove someone from project then it should remove from the asset which are attach to that project so for that i written code in project’s “after_save”

in below snippet only country field is updating but _assign field not,
not able to understand why this is happening?

                    frappe.db.set_value('Asset', asset.name, {