The issue is known to happen with 5.1.4 and 5.1.5, but the most important factor is using mongo ruby driver 2.3.0.

As seen in the documentation :

"You can define indexes on embedded document fields as well."

https://docs.mongodb.com/ruby-driver/master/tutorials/5.1.0/mongoid-indexes/

This does not work anymore.

You can easily break the RSpec suite by adding the following line to spec/app/models/person.rb :

index "addresses.street" => 1

The RSpec suite will then fail at multiple places, throwing BSON::String::IllegalKey exceptions :

Failure/Error: collection.indexes.create_one(key, options)

BSON::String::IllegalKey:
'addresses.street' is an illegal key in MongoDB. Keys may not start with '$' or contain a '.'

You can also trigger the bug by executing the create_indexes rake task.

The only workaround is to use mongo driver < 2.3.0.