I obtained this error when instance any one model like;

Model.where(query).first
Model.find_by(query)
In mongoid 6.0.0 works fine.

NoMethodError (undefined method first for false:FalseClass):

app/controllers/api/v1/sessions_controller.rb:4:in `create'
Started GET "/prueba" for 127.0.0.1 at 2016-11-03 15:22:59 +0100
Processing by PruebaController#hola as HTML
Completed 500 Internal Server Error in 68ms

NoMethodError - undefined method `first' for false:FalseClass:
mongoid (6.0.2) lib/mongoid/contextual/mongo.rb:250:in `block in first'
mongoid (6.0.2) lib/mongoid/contextual/mongo.rb:513:in `try_cache'
mongoid (6.0.2) lib/mongoid/contextual/mongo.rb:246:in `first'
mongoid (6.0.2) lib/mongoid/contextual.rb:20:in `first'
mongoid (6.0.2) lib/mongoid/findable.rb:148:in `first'
app/controllers/prueba_controller.rb:3:in `hola'
actionpack (5.0.0.1) lib/action_controller/metal/basic_implicit_render.rb:4:in `send_action'
actionpack (5.0.0.1) lib/abstract_controller/base.rb:188:in `process_action'
actionpack (5.0.0.1) lib/action_controller/metal/rendering.rb:30:in `process_action'
actionpack (5.0.0.1) lib/abstract_controller/callbacks.rb:20:in `block in process_action'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:126:in `call'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:506:in `block (2 levels) in compile'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:455:in `call'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:101:in `_run_callbacks_'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:750:in `_run_process_action_callbacks'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:90:in `run_callbacks'
actionpack (5.0.0.1) lib/abstract_controller/callbacks.rb:19:in `process_action'
actionpack (5.0.0.1) lib/action_controller/metal/rescue.rb:20:in `process_action'
actionpack (5.0.0.1) lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
activesupport (5.0.0.1) lib/active_support/notifications.rb:164:in `block in instrument'
activesupport (5.0.0.1) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (5.0.0.1) lib/active_support/notifications.rb:164:in `instrument'
actionpack (5.0.0.1) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
actionpack (5.0.0.1) lib/action_controller/metal/params_wrapper.rb:248:in `process_action'
actionpack (5.0.0.1) lib/abstract_controller/base.rb:126:in `process'
actionview (5.0.0.1) lib/action_view/rendering.rb:30:in `process'
actionpack (5.0.0.1) lib/action_controller/metal.rb:190:in `dispatch'
actionpack (5.0.0.1) lib/action_controller/metal.rb:262:in `dispatch'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:50:in `dispatch'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:32:in `serve'
actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:39:in `block in serve'
actionpack (5.0.0.1) lib/action_dispatch/journey/router.rb:26:in `serve'
actionpack (5.0.0.1) lib/action_dispatch/routing/route_set.rb:725:in `call'
warden (1.2.6) lib/warden/manager.rb:35:in `block in call'
warden (1.2.6) lib/warden/manager.rb:34:in `call'
rack (2.0.1) lib/rack/etag.rb:25:in `call'
rack (2.0.1) lib/rack/conditional_get.rb:25:in `call'
rack (2.0.1) lib/rack/head.rb:12:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:38:in `block in call'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:97:in `_run_callbacks_'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:750:in `_run_call_callbacks'
activesupport (5.0.0.1) lib/active_support/callbacks.rb:90:in `run_callbacks'
actionpack (5.0.0.1) lib/action_dispatch/middleware/callbacks.rb:36:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/remote_ip.rb:79:in `call'
better_errors (2.1.1) lib/better_errors/middleware.rb:84:in `protected_app_call'
better_errors (2.1.1) lib/better_errors/middleware.rb:79:in `better_errors_call'
better_errors (2.1.1) lib/better_errors/middleware.rb:57:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/debug_exceptions.rb:49:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/show_exceptions.rb:31:in `call'
railties (5.0.0.1) lib/rails/rack/logger.rb:36:in `call_app'
railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `block in call'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `block in tagged'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:26:in `tagged'
activesupport (5.0.0.1) lib/active_support/tagged_logging.rb:70:in `tagged'
railties (5.0.0.1) lib/rails/rack/logger.rb:24:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/request_id.rb:24:in `call'
rack (2.0.1) lib/rack/runtime.rb:22:in `call'
activesupport (5.0.0.1) lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/executor.rb:12:in `call'
actionpack (5.0.0.1) lib/action_dispatch/middleware/static.rb:136:in `call'
rack (2.0.1) lib/rack/sendfile.rb:111:in `call'
rack-cors (0.4.0) lib/rack/cors.rb:80:in `call'
railties (5.0.0.1) lib/rails/engine.rb:522:in `call'
puma (3.6.0) lib/puma/configuration.rb:225:in `call'
puma (3.6.0) lib/puma/server.rb:578:in `handle_request'
puma (3.6.0) lib/puma/server.rb:415:in `process_client'
puma (3.6.0) lib/puma/server.rb:275:in `block in run'
puma (3.6.0) lib/puma/thread_pool.rb:116:in `block in spawn_thread'