update trigger

update trigger written as follows is not working and we are not able to update the record because of the foreign key constraints violation.pls help and
do send the reply at [email protected]

Create Trigger <trigname> on <tablename>
for update
as

declare @key_after nvarchar,@key_before nvarchar

if UPDATE( <fieldname> )

begin

Select @key_before = <fieldname> from deleted
Select @key_after = <fieldname> from inserted

Update <fk_table_name>
set <fieldname> = @key_after
where <fieldname> = @key_before

end