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
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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