irb(main):001:0> a=Mongo::Client.new(['127.0.0.1:27100'])
D, [2018-10-11T16:41:32.400373 #29747] DEBUG -- : MONGODB | EVENT: #<TopologyOpening topology=Unknown[]>
D, [2018-10-11T16:41:32.400495 #29747] DEBUG -- : MONGODB | Topology type 'unknown' initializing.
D, [2018-10-11T16:41:32.400741 #29747] DEBUG -- : MONGODB | EVENT: #<ServerOpening address=127.0.0.1:27100 topology=Unknown[127.0.0.1:27100]>
D, [2018-10-11T16:41:32.400791 #29747] DEBUG -- : MONGODB | Server 127.0.0.1:27100 initializing.
D, [2018-10-11T16:41:32.402680 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerDescriptionChanged>
D, [2018-10-11T16:41:32.402780 #29747] DEBUG -- : MONGODB | Server description for 127.0.0.1:27100 changed from 'unknown' to 'unknown'.
D, [2018-10-11T16:41:32.402961 #29747] DEBUG -- : MONGODB | EVENT: #<ServerOpening address=localhost:27100 topology=Unknown[127.0.0.1:27100]>
D, [2018-10-11T16:41:32.403006 #29747] DEBUG -- : MONGODB | Server localhost:27100 initializing.
D, [2018-10-11T16:41:32.404620 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerDescriptionChanged>
D, [2018-10-11T16:41:32.404701 #29747] DEBUG -- : MONGODB | Server description for localhost:27100 changed from 'unknown' to 'primary'.
D, [2018-10-11T16:41:32.404874 #29747] DEBUG -- : MONGODB | EVENT: #<ServerOpening address=localhost:27101 topology=Unknown[127.0.0.1:27100]>
D, [2018-10-11T16:41:32.404918 #29747] DEBUG -- : MONGODB | Server localhost:27101 initializing.
D, [2018-10-11T16:41:32.406172 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerDescriptionChanged>
D, [2018-10-11T16:41:32.406244 #29747] DEBUG -- : MONGODB | Server description for localhost:27101 changed from 'unknown' to 'secondary'.
D, [2018-10-11T16:41:32.406459 #29747] DEBUG -- : MONGODB | EVENT: #<ServerOpening address=localhost:27102 topology=Unknown[127.0.0.1:27100]>
D, [2018-10-11T16:41:32.406503 #29747] DEBUG -- : MONGODB | Server localhost:27102 initializing.
D, [2018-10-11T16:41:32.407728 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerDescriptionChanged>
D, [2018-10-11T16:41:32.407797 #29747] DEBUG -- : MONGODB | Server description for localhost:27102 changed from 'unknown' to 'secondary'.
D, [2018-10-11T16:41:32.407969 #29747] DEBUG -- : MONGODB | EVENT: #<TopologyChanged prev=Unknown[127.0.0.1:27100] new=ReplicaSetNoPrimary>
D, [2018-10-11T16:41:32.408028 #29747] DEBUG -- : MONGODB | Topology type 'unknown' changed to type 'replicasetnoprimary'.
D, [2018-10-11T16:41:32.408577 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerClosed>
D, [2018-10-11T16:41:32.408633 #29747] DEBUG -- : MONGODB | Server 127.0.0.1:27100 connection closed.
D, [2018-10-11T16:41:32.408818 #29747] DEBUG -- : MONGODB | EVENT: #<TopologyChanged prev=ReplicaSetNoPrimary new=ReplicaSetWithPrimary>
D, [2018-10-11T16:41:32.408872 #29747] DEBUG -- : MONGODB | Topology type 'replicasetnoprimary' changed to type 'replicasetwithprimary'.
=> #<Mongo::Client:0x47314406949200 cluster=#<Cluster topology=ReplicaSetWithPrimary servers=[#<Server address=localhost:27100 PRIMARY replica_set=ruby-driver-rs>,#<Server address=localhost:27101 SECONDARY replica_set=ruby-driver-rs>,#<Server address=localhost:27102 SECONDARY replica_set=ruby-driver-rs>]>>
irb(main):002:0> b=a.dup
=> #<Mongo::Client:0x47314406663380 cluster=#<Cluster topology=ReplicaSetWithPrimary servers=[#<Server address=localhost:27100 PRIMARY replica_set=ruby-driver-rs>,#<Server address=localhost:27101 SECONDARY replica_set=ruby-driver-rs>,#<Server address=localhost:27102 SECONDARY replica_set=ruby-driver-rs>]>>
irb(main):003:0> b.database
=> nil
irb(main):004:0> a.database
=> #<Mongo::Database:0x47314406899640 name=admin>
irb(main):005:0> D, [2018-10-11T16:41:42.411746 #29747] DEBUG -- : MONGODB | EVENT: #<Mongo::Monitoring::Event::ServerDescriptionChanged>
D, [2018-10-11T16:41:42.411877 #29747] DEBUG -- : MONGODB | Server description for 127.0.0.1:27100 changed from 'unknown' to 'unknown'.
b['test']
Traceback (most recent call last):
        3: from /home/w/.rbenv/versions/2.5.1/bin/irb:11:in `<main>'
        2: from (irb):5
        1: from /home/w/apps/ruby-driver/lib/mongo/client.rb:139:in `[]'
NoMethodError (undefined method `[]' for nil:NilClass)
irb(main):006:0> a['test']
=> #<Mongo::Collection:0x47314405367640 namespace=admin.test>