ActionController::UnknownFormat with rspec and post request

You're using json format in controller, but you're passing format: :js in the test.

It should be:

post sensors_path, sensor: @sensor_attributes, format: :json

This is for future readers. Somehow the answer did not help me.

Writing like following worked for me -

params = {
    sensor: { 
       name: "Testname", 
       description: "This is a Test-Description." 
    },
    format: :json
  }

  post sensors_path, params