Problem Statement/Rationale

When restoring a collMod oplog that converts a normal index to TTL index, mongorestore first looks up the index from the cluster and then attempts to find the expireAfterSeconds field from it. This results in a failure because the field doesn't exist in the index spec.

Steps to Reproduce

1. Create a normal index on the cluster without the TTL field
2. Run mongodump and dump with oplog
3. Immediately convert the index to TTL using collMod
4. Use mongorestore to restore the oplogs containing the collMod oplog

Expected Results

Mongorestore should succeed with the normal index being converted to TTL

Actual Results

Mongorestore fails with error:

Failed: restore error: error applying oplog: could not handle collMod on test.collmod_convert_to_ttl: missing "expireAfterSeconds" in matching index: &{map[name:a_1 v:2] [{a 1}] []}

Additional Notes

The bug was found in a passthrough test (link)