When using Mongoid in heavily concurrent parts of my app, I often run into this issue. Calling document.set(field_name: 1) sometimes triggers this error:

NoMethodError: undefined method `reject' for nil:NilClass

• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/clients/options.rb" line 54 in mongo_client
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/clients.rb" line 94 in mongo_client
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/clients.rb" line 90 in collection
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/persistable.rb" line 212 in persist_atomic_operations
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/persistable.rb" line 195 in persist_or_delay_atomic_operation
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/persistable.rb" line 146 in prepare_atomic_operation
• File "/home/app/vendor/bundle/jruby/2.2.0/gems/mongoid-5.0.1/lib/mongoid/persistable/settable.rb" line 23 in set

Calling document.with({}).set(field_name: 1) seems to fix the issue.