I have postgresql table that has somekind of unique constraint.

I have ruby script that will update this table. The ruby script should be able to switch to UPDATE instead of INSERT when this kind of error occured:
PGError: ERROR: duplicate key value violates unique constraint

CMIIW, sequel seems cannot catch this exception??

Anyway,sample code below is something that I would like to see to work but apparently not:

@myarray.each do |x|
        fresh_ds = DB["INSERT INTO mytable (id, col_foo) values ('#{x}' ,'#{myhash[x]}')"] 
        result = fresh_ds.insert

        catch Sequel::Error do

            fresh_ds = DB["UPDATE mytable set col_foo = '#{myhash[x]} where id = #{x}"]
            result = fresh_ds.update

        end

end

Maybe my ruby code is wrong or I missed something I don't know.
Any solution?
Thanks.

UPDATE: code below works, the error is caught when unique constraint is violated.

@myarray.each do |x|
    begin
      # INSERT CODE
    rescue Sequel::Error
      # UPDATE CODE
    end

end