The lists: Backlog, and Done cannot be destroyed

app/models/board.rb

 class Board < ActiveRecord::Base
   belongs_to :project
-  has_many :lists, -> { order(:list_type, :position) }, dependent: :destroy
+  has_many :lists, -> { order(:list_type, :position) }, dependent: :delete_all
   validates :project, presence: true
 end

app/models/list.rb

@@ -10,7 +10,15 @@ class List < ActiveRecord::Base
   delegate :name, to: :label, allow_nil: true, prefix: true
+  before_destroy :can_be_destroyed, unless: :label?
+
   def title
     label? ? label_name : list_type.humanize
   end
+
+  private
+
+  def can_be_destroyed
+    false
+  end
 end

spec/models/board_spec.rb

@@ -3,7 +3,7 @@ require 'rails_helper'
 describe Board do
   describe 'relationships' do
     it { is_expected.to belong_to(:project) }
-    it { is_expected.to have_many(:lists).order(list_type: :asc, position: :asc).dependent(:destroy) }
+    it { is_expected.to have_many(:lists).order(list_type: :asc, position: :asc).dependent(:delete_all) }
   end
   describe 'validations' do

spec/models/list_spec.rb

@@ -31,6 +31,27 @@ describe List do
       it { is_expected.not_to validate_presence_of(:position) }
     end
   end
+
+  describe '#destroy' do
+    it 'can be destroyed when when list_type is set to label' do
+      subject = create(:label_list)
+
+      expect(subject.destroy).to be_truthy
+    end
+
+    it 'can not be destroyed when list_type is set to backlog' do
+      subject = create(:backlog_list)
+
+      expect(subject.destroy).to be_falsey
+    end
+
+    it 'can not be destroyed when when list_type is set to done' do
+      subject = create(:done_list)
+
+      expect(subject.destroy).to be_falsey
+    end
+  end
+
   describe '#title' do
     it 'returns label name when list_type is set to label' do
       subject.list_type = :label