@halostatue has answered your first question. Now as to your other question – about the keys. I’ve had success with the custom_base library. It generates code for a custom BaseN encoder/decoder.

defmodule MyBase do
  use CustomBase, ~c(0123456789abcdefghijklmnopqrstuvwxyz) # <-- you can customize these chars
                                                           # notice it is NOT a string!

  def encode(integer)

  def decode(binary)

  def decode!(binary)
end

The nice thing about this one is that you can continue to use integer keys (so actually you wouldn’t have to change much). When looking up the coupon code, you’d just decode it.

In this example, our coupon code is “jjp” which would come from the row id 25333:

iex(1)> MyBase.encode(25333)
"jjp"
iex(2)> MyBase.decode!("jjp")
25333

There might be concern about someone finding this pattern, so you could use a different order for the characters:

~c(a3oqmcylhd0w81v642eipzr9b5g7tufknsjx)

Or, if these coupon codes will be spoken over the phone, you might remove certain characters that can be misheard (i.e. b, c, d, e, p, t, v, and z are easy to mishear because they have the /iː/ sound at the end).

~c(0123456789adfhijklmoqruwxy)

Of course keeping in mind that you can shuffle the characters to avoid any exploitation/prediction of the next coupon code in a sequence.

Finally - you have to be careful with this method. You will have to maintain it for the life of the youngest coupon code generated using this method.