ruby.rb

So I'm trying to set the value of a primary key in my model...I'm thinking rails should be doing this on it's own but I'm not sure. My question is what should I be putting in place of "DEFAULT"?

tabel schema
  create_table "Model_Labels", primary_key: "labelid", force: :cascade do |t|
    t.string "name", limit: 255
  end

Portion of code where I want to set the value of "labelid"

			model_labels = Model_Labels.new

			model_labels.labelid 	= DEFAULT
			model_labels.name 	= item['name']
			model_labels.save
			label_id = model_labels.labelid 

NOTE label_id get's called latter in the code