CentOS 7
Ruby 2.6.3
Rails (5.2.2)
Mongoid (7.0.2)

$ mongo --version
MongoDB shell version v4.0.9
git version: fc525e2d9b0e4bceff5c2201457e564362909765
OpenSSL version: OpenSSL 1.0.1e-fips 11 Feb 2013
allocator: tcmalloc
modules: none
build environment:
    distmod: rhel70
    distarch: x86_64
    target_arch: x86_64

production:
  clients:
    default:
      database: mydb_production
      hosts:
        - localhost:27017
      username: myuser
      password: "mypass"
      options:
        auth_mech: :scram
        auth_source: admin

D, [2019-05-27T20:01:58.272760 #27444] DEBUG -- : MONGODB | EVENT: #<TopologyOpening topology=Unknown[]>
D, [2019-05-27T20:01:58.272934 #27444] DEBUG -- : MONGODB | Topology type 'unknown' initializing.
D, [2019-05-27T20:01:58.273748 #27444] DEBUG -- : MONGODB | EVENT: #<TopologyChanged prev=Unknown[] new=Unknown[localhost:27017]>
D, [2019-05-27T20:01:58.273848 #27444] DEBUG -- : MONGODB | Topology type 'Unknown' changed to type 'Unknown'.
D, [2019-05-27T20:01:58.274028 #27444] DEBUG -- : MONGODB | EVENT: #<ServerOpening address=localhost:27017 topology=Unknown[localhost:27017]>
D, [2019-05-27T20:01:58.274091 #27444] DEBUG -- : MONGODB | Server localhost:27017 initializing.
D, [2019-05-27T20:01:58.365308 #27444] DEBUG -- : MONGODB | EVENT: #<ServerDescriptionChanged address=localhost:27017 topology=Single[localhost:27017] prev=#<Mongo::Server:Description:0x70049014837980 config={} average_round_trip_time=> new=#<Mongo::Server:Description:0x70048970460940 config={"ismaster"=>true, "maxBsonObjectSize"=>16777216, "maxMessageSizeBytes"=>48000000, "maxWriteBatchSize"=>100000, "localTime"=>2019-05-28 00:01:58 UTC, "logicalSessionTimeoutMinutes"=>30, "minWireVersion"=>0, "maxWireVersion"=>7, "readOnly"=>false, "ok"=>1.0} average_round_trip_time=0.088107942>>
D, [2019-05-27T20:01:58.365596 #27444] DEBUG -- : MONGODB | Server description for localhost:27017 changed from 'unknown' to 'standalone'.
D, [2019-05-27T20:01:58.365843 #27444] DEBUG -- : MONGODB | EVENT: #<TopologyChanged prev=Unknown[localhost:27017] new=Single[localhost:27017]>
D, [2019-05-27T20:01:58.366024 #27444] DEBUG -- : MONGODB | Topology type 'Unknown' changed to type 'Single'.

$ rails c production
Running via Spring preloader in process 3131
Loading production environment (Rails 5.2.2)
2.6.3 :001 > User.first
Traceback (most recent call last):
        1: from (irb):1
NoMethodError (undefined method `encode' for nil:NilClass)