Update mongodb demo

Src/Asp.NetCore2/MongoDbTest/UnitTest/QueryLeftJoin.cs

@@ -0,0 +1,50 @@
+using MongoDB.Bson;
+using SqlSugar.MongoDb;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MongoDbTest 
+{
+    internal class QueryLeftJoin
+    {
+        internal static void Init()
+        {
+            var db = DbHelper.GetNewDb();
+            db.DbMaintenance.TruncateTable<School, School>(); 
+            // 添加学校数据
+            var school = new School { Name = "TestSchool" };
+            var ids=db.Insertable(school).ExecuteReturnPkList<string>(); 
+            // 添加学生数据，SchoolId 关联学校
+            var student = new Student { Name = "TestStudent", SchoolId = ids.Last() }; 
+            db.Insertable(student).ExecuteCommand();
+            // 添加学生数据，SchoolId 关联学校并且没有学校
+            var student2 = new Student { Name = "TestStudent", SchoolId =
+               ObjectId.GenerateNewId().ToString()
+            }; 
+            db.Insertable(student2).ExecuteCommand();
+
+            //var list=db.Queryable<Student>()
+            //    .LeftJoin<School>((x, y) => x.SchoolId == y.Id)
+            //    .Where((x,y)=>x.Name =="a"||y.Name=="a")
+            //    .Select(x => new
+            //    {
+            //        id=x.Name
+            //    }).ToList();
+        }
+        [SqlSugar.SugarTable("UnitStudent123131")]
+        public class Student : MongoDbBase
+        {
+            public string Name { get; set; }
+
+            public string SchoolId { get; set; }
+        }
+        [SqlSugar.SugarTable("UnitSchool123131")]
+        public class School : MongoDbBase
+        {
+            public string Name { get; set; }
+        }
+    }
+}