https://github.com/mongoid/moped/issues/237

moped_16550_error

12:12:27 web.1  | started with pid 6842
12:12:29 web.1  | I, [2013-12-03T12:12:29.239032 #6842]  INFO -- : Refreshing Gem list
12:12:33 web.1  | /moped-2.0.0.beta4/lib/moped/operation/read.rb:50:in `block in execute': The operation: #<Moped::Protocol::Query (Moped::Errors::QueryFailure)
12:12:33 web.1  |   @length=51
12:12:33 web.1  |   @request_id=5
12:12:33 web.1  |   @response_to=0
12:12:33 web.1  |   @op_code=2004
12:12:33 web.1  |   @flags=[]
12:12:33 web.1  |   @full_collection_name="app19591881.roles"
12:12:33 web.1  |   @skip=0
12:12:33 web.1  |   @limit=0
12:12:33 web.1  |   @selector={}
12:12:33 web.1  |   @fields=nil>
12:12:33 web.1  | failed with error 16550: "not authorized for query on app19591881.roles"
12:12:33 web.1  | 
12:12:33 web.1  | See https://github.com/mongodb/mongo/blob/master/docs/errors.md
12:12:33 web.1  | for details about this error.
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:595:in `[]'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:595:in `block (2 levels) in flush'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:594:in `map'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:594:in `block in flush'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:618:in `block in logging'
12:12:33 web.1  |   from /activesupport-4.0.1/lib/active_support/notifications.rb:159:in `block in instrument'
12:12:33 web.1  |   from /activesupport-4.0.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
12:12:33 web.1  |   from /activesupport-4.0.1/lib/active_support/notifications.rb:159:in `instrument'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/instrumentable.rb:31:in `instrument'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:617:in `logging'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:588:in `flush'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:392:in `process'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/operation/read.rb:48:in `execute'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:649:in `read'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:412:in `query'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/cursor.rb:137:in `block in load_docs'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/cluster.rb:243:in `block in with_primary'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:205:in `block in ensure_primary'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/executable.rb:25:in `execute'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/node.rb:204:in `ensure_primary'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/cluster.rb:242:in `with_primary'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/read_preference/primary.rb:55:in `block in with_node'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/read_preference/selectable.rb:65:in `call'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/read_preference/selectable.rb:65:in `with_retry'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/read_preference/primary.rb:54:in `with_node'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/cursor.rb:136:in `load_docs'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/cursor.rb:27:in `each'
12:12:33 web.1  |   from /moped-2.0.0.beta4/lib/moped/query.rb:77:in `each'
12:12:33 web.1  |   from /mongoid-f8b439dd9a59/lib/mongoid/contextual/mongo.rb:123:in `block in each'
12:12:33 web.1  |   from /mongoid-f8b439dd9a59/lib/mongoid/contextual/mongo.rb:650:in `selecting'
12:12:33 web.1  |   from /mongoid-f8b439dd9a59/lib/mongoid/contextual/mongo.rb:122:in `each'
12:12:33 web.1  |   from /mongoid-f8b439dd9a59/lib/mongoid/contextual.rb:19:in `each'

Having the same issue while running rails 4 and ruby 2