Add the guard for search index of discount collection

[mikehquan19]

Overview

Add the guard to only create the search index for discount collection if it doesn't exist yet. The guard should be able to identify the search index at the field level, which means it compares each field of the schema used for the combination to create the search index.

[mikehquan19]

@bvaic Let me close this once I've reviewed your PRs :).

[bvaic]

Whoops sorry I didn't mean to do that