Here’s my flow for debugging a misbehaving controller. Imagine that there’s a form at app/views/messages/new.html.erb that is posting to MessagesController#create. The parameters aren’t being passed through from the form to the controller, and the model is raising a validation error saying that a field is blank – even though it’s clearly not.

Step 0: Does restarting the server fix the problem?

Step 1: Check the form

• Is the attribute defined in the correct place?
• Is it using the form builder helper (i.e. f.text_field) or is it using text_field_tag? If it’s supposed to be params[:message][:body] then it should be using the form builder. If not, then it should be using text_field_tag or similar.
• Unlikely, but still possible: Is the field disabled before submission of the form? Disabled fields are not passed through as parameters.

Step 2: Check the controller

• Is the route to the controller defined correctly in config/routes.rb?
• Is the controller set up to receive the right parameters?
• Did I spell ‘message’ with the correct number of ‘s’s within params[:message]?
• Are the parameters from the form permitted using strong_parameters?
• Are all the parameters from the form permitted?
• Did I spell the parameters correctly?
• What does the log output say for this controller? Does it contain the parameters I want with the values I want?
• Can I place a binding.pry statement at the top of the action and print out params to see the correct parameters there too?

Step 3: Check the model

• If using attr_accessible, are the attribute marked as accessible?
• Did I accidentally use attr_accessor when I meant to use attr_accessible?
• Did I define a setter method for this attribute accidentally that doesn’t actually set the attribute?
• Is there a callback that unsets this parameter somehow?
• If it’s nested attributes that are being sent through, go check the nested model for the above.