Add service to remove a list from board

app/services/boards/lists/destroy_service.rb

+module Boards
+  module Lists
+    class DestroyService
+      def initialize(project, params = {})
+        @board  = project.board
+        @params = params.dup
+      end
+
+      def execute
+        list.with_lock do
+          reorder_higher_lists
+          remove_list
+        end
+      end
+
+      private
+
+      attr_reader :board, :params
+
+      def list
+        @list ||= board.lists.find(params[:list_id])
+      end
+
+      def reorder_higher_lists
+        board.lists.where('position > ?',  list.position)
+             .update_all('position = position - 1')
+      end
+
+      def remove_list
+        list.destroy
+      end
+    end
+  end
+end

spec/services/boards/lists/destroy_service_spec.rb

+require 'spec_helper'
+
+describe Boards::Lists::DestroyService, services: true do
+  describe '#execute' do
+    let(:project) { create(:project_with_board) }
+    let(:board)   { project.board }
+
+    it 'removes list from board' do
+      list = create(:list, board: board)
+      service = described_class.new(project, list_id: list.id)
+
+      expect { service.execute }.to change(board.lists, :count).by(-1)
+    end
+
+    it 'decrements position of higher lists' do
+      list1 = create(:list, board: board, position: 1)
+      list2 = create(:list, board: board, position: 2)
+      list3 = create(:list, board: board, position: 3)
+      list4 = create(:list, board: board, position: 4)
+      list5 = create(:list, board: board, position: 5)
+
+      described_class.new(project, list_id: list2.id).execute
+
+      expect(list1.reload.position).to eq 1
+      expect(list3.reload.position).to eq 2
+      expect(list4.reload.position).to eq 3
+      expect(list5.reload.position).to eq 4
+    end
+  end
+end