@appliance = appliance.new(params[:appliance])
@appliance.manufacturer_id = params[:manufacturer_id]

I would do:

@appliance = @manufacturer.appliances.build(params[:appliance])

It’s a little method thrown into ActiveRecord for just this purpose. Similarly when you are doing you “edit” and “show” methods you can do:

@appliance = @manufacturer.appliances.find(params[:id])

This ensures that the appliance you are retrieving actually belongs to the parent manufacturer.