Bootstrap 4 TreeView

The tree is using bootstrap 4

<html>
<head>
    <meta charset="utf-8" />
    <title>Bootstrap 4 TreeView</title>
    <script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
    <script src="https://unpkg.com/gijgo@1.9.14/js/gijgo.min.js" type="text/javascript"></script>
    <link href="https://unpkg.com/gijgo@1.9.14/css/gijgo.min.css" rel="stylesheet" type="text/css" />
</head>
<body>
    <div class="container-fluid">
        <div id="tree"></div>
    </div>
    <script type="text/javascript">
            $(document).ready(function () {
                $('#tree').tree({
                    uiLibrary: 'bootstrap4',
                    dataSource: '/Locations/Get',
                    primaryKey: 'id',
                    imageUrlField: 'flagUrl'
                });
            });
    </script>
</body>
</html>

using Gijgo.Asp.NET.Examples.Models.Entities;
using System.Collections.Generic;
using System.Linq;
using System.Web.Mvc;

namespace Gijgo.Asp.NET.Examples.Controllers
{
    public class LocationsController : Controller
    {
        public JsonResult Get(string query)
        {
            List<Location> locations;
            List<Models.DTO.Location> records;
            using (ApplicationDbContext context = new ApplicationDbContext())
            {
                locations = context.Locations.ToList();

if (!string.IsNullOrWhiteSpace(query))
                {
                    locations = locations.Where(q => q.Name.Contains(query)).ToList();
                }

records = locations.Where(l => l.ParentID == null).OrderBy(l => l.OrderNumber)
                    .Select(l => new Models.DTO.Location
                    {
                        id = l.ID,
                        text = l.Name,
                        @checked = l.Checked,
                        population = l.Population,
                        flagUrl = l.FlagUrl,
                        children = GetChildren(locations, l.ID)
                    }).ToList();
            }

return this.Json(records, JsonRequestBehavior.AllowGet);
        }

public JsonResult LazyGet(int? parentId)
        {
            List<Location> locations;
            List<Models.DTO.Location> records;
            using (ApplicationDbContext context = new ApplicationDbContext())
            {
                locations = context.Locations.ToList();

records = locations.Where(l => l.ParentID == parentId).OrderBy(l => l.OrderNumber)
                    .Select(l => new Models.DTO.Location
                    {
                        id = l.ID,
                        text = l.Name,
                        @checked = l.Checked,
                        population = l.Population,
                        flagUrl = l.FlagUrl,
                        hasChildren = locations.Any(l2 => l2.ParentID == l.ID)
                    }).ToList();
            }

return this.Json(records, JsonRequestBehavior.AllowGet);
        }

private List<Models.DTO.Location> GetChildren(List<Location> locations, int parentId)
        {
            return locations.Where(l => l.ParentID == parentId).OrderBy(l => l.OrderNumber)
                .Select(l => new Models.DTO.Location
                {
                    id = l.ID,
                    text = l.Name,
                    population = l.Population,
                    flagUrl = l.FlagUrl,
                    @checked = l.Checked,
                    children = GetChildren(locations, l.ID)
                }).ToList();
        }

[HttpPost]
        public JsonResult SaveCheckedNodes(List<int> checkedIds)
        {
            if (checkedIds == null)
            {
                checkedIds = new List<int>();
            }
            using (ApplicationDbContext context = new ApplicationDbContext())
            {
                var locations = context.Locations.ToList();
                foreach (var location in locations)
                {
                    location.Checked = checkedIds.Contains(location.ID);
                }
                context.SaveChanges();
            }

return this.Json(true);
        }

[HttpPost]
        public JsonResult ChangeNodePosition(int id, int parentId, int orderNumber)
        {
            using (ApplicationDbContext context = new ApplicationDbContext())
            {
                var location = context.Locations.First(l => l.ID == id);

var newSiblingsBelow = context.Locations.Where(l => l.ParentID == parentId && l.OrderNumber >= orderNumber);
                foreach (var sibling in newSiblingsBelow)
                {
                    sibling.OrderNumber = sibling.OrderNumber + 1;
                }

var oldSiblingsBelow = context.Locations.Where(l => l.ParentID == location.ParentID && l.OrderNumber > location.OrderNumber);
                foreach (var sibling in oldSiblingsBelow)
                {
                    sibling.OrderNumber = sibling.OrderNumber - 1;
                }

location.ParentID = parentId;
                location.OrderNumber = orderNumber;

context.SaveChanges();
            }

return this.Json(true);
        }

public JsonResult GetCountries(string query)
        {
            List<Models.DTO.Location> records;
            using (ApplicationDbContext context = new ApplicationDbContext())
            {
                records = context.Locations.Where(l => l.Parent != null && l.Parent.ParentID == null)
                    .Select(l => new Models.DTO.Location
                    {
                        id = l.ID,
                        text = l.Name,
                        @checked = l.Checked,
                        population = l.Population,
                        flagUrl = l.FlagUrl
                    }).ToList();
            }

return this.Json(records, JsonRequestBehavior.AllowGet);
        }
    }
}

;