This error happens only in testing environment and only in Rails 5.2.0. When reverting to Rails 5.1.6 error doesn't appear. So far I also didn't encounter any errors in my development and staging environment.

Error backtrace:
NoMethodError: private method `fields' called for #<DcPolicyRole:0x00000000033410e0>
Did you mean? fields=
fields?
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/validatable/localizable.rb:19:in `validate_each'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validator.rb:152:in `block in validate'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validator.rb:149:in `each'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validator.rb:149:in `validate'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:426:in `block in make_lambda'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:198:in `block (2 levels) in halting'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:606:in `block (2 levels) in default_terminator'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:605:in `catch'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:605:in `block in default_terminator'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:199:in `block in halting'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:513:in `block in invoke_before'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:513:in `each'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:513:in `invoke_before'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:131:in `run_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/interceptable.rb:132:in `run_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:816:in `_run_validate_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validations.rb:409:in `run_validations!'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validations/callbacks.rb:118:in `block in run_validations!'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:98:in `run_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/interceptable.rb:132:in `run_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:816:in `_run_validation_callbacks'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validations/callbacks.rb:118:in `run_validations!'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validations.rb:339:in `valid?'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/validatable.rb:97:in `valid?'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/activemodel-5.2.0/lib/active_model/validations.rb:376:in `invalid?'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/persistable/creatable.rb:117:in `prepare_insert'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/persistable/creatable.rb:23:in `insert'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/persistable/creatable.rb:180:in `block in create!'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/threaded/lifecycle.rb:161:in `_creating'
/home/rails/Testiranje/vendor/ruby/2.4.0/gems/mongoid-6.4.0/lib/mongoid/persistable/creatable.rb:175:in `create!'
/home/rails/drg_cms/test/fixtures/drg_cms_test_data.rb:6:in `drg_cms_test_data_fill'

Line 6 of drg_cms_test_data.rb:
sa_role = DcPolicyRole.create!(name: 'superadmin', system_name: 'superadmin')

by
TheR