I am not able to create indexes for embedded documents. I.e I have Contact which has many email addresses and i want to create index on "emails.address". I can do this directly via Mongo console (and it was possible in older versions of Mongo/Mongoid) but now I am getting following error:
'emails.address' is an illegal key in MongoDB. Keys may not start with '$' or contain a '.'.

Backtrace:
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/string.rb:69:in `to_bson_key'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/symbol.rb:74:in `to_bson_key'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:45:in `block in to_bson'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:43:in `each'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:43:in `to_bson'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:46:in `block in to_bson'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:43:in `each'
/usr/lib/ruby/gems/2.3.0/gems/bson-4.1.1/lib/bson/hash.rb:43:in `to_bson'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/serializers.rb:163:in `serialize'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:202:in `block (2 levels) in serialize_fields'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:200:in `each'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:200:in `block in serialize_fields'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:197:in `each'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:197:in `serialize_fields'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/protocol/message.rb:100:in `serialize'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:187:in `block in write'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:186:in `each'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:186:in `write'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:158:in `deliver'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:105:in `block in dispatch'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/monitoring/publishable.rb:47:in `publish_command'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection.rb:104:in `dispatch'
/usr/lib/ruby/gems/2.3.0/gems/newrelic_mongodb-0.1.2/lib/newrelic_mongodb/instrumentation.rb:33:in `block in logging_with_newrelic_trace'
/usr/lib/ruby/gems/2.3.0/gems/newrelic_rpm-3.17.0.325/lib/new_relic/agent/datastores.rb:111:in `wrap'
/usr/lib/ruby/gems/2.3.0/gems/newrelic_mongodb-0.1.2/lib/newrelic_mongodb/instrumentation.rb:32:in `logging_with_newrelic_trace'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/operation/write/gle.rb:31:in `block in execute_message'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server/connection_pool.rb:107:in `with_connection'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/server.rb:231:in `with_connection'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/operation/write/gle.rb:28:in `execute_message'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/operation/write/write_command_enabled.rb:39:in `execute'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/index/view.rb:154:in `create_many'
/usr/lib/ruby/gems/2.3.0/gems/mongo-2.3.0/lib/mongo/index/view.rb:129:in `create_one'
/usr/lib/ruby/gems/2.3.0/gems/mongoid-5.1.4/lib/mongoid/indexable.rb:37:in `block in create_indexes'
/usr/lib/ruby/gems/2.3.0/gems/mongoid-5.1.4/lib/mongoid/indexable.rb:31:in `each'
/usr/lib/ruby/gems/2.3.0/gems/mongoid-5.1.4/lib/mongoid/indexable.rb:31:in `create_indexes'