Update mongodb

Src/Asp.NetCore2/SqlSugar.MongoDbCore/MongoDb/SqlBuilder/MongoDbInsertBuilder.cs

@@ -2,6 +2,7 @@
 using MongoDB.Bson;
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization;
+using MongoDB.Driver;
 using NetTaste;
 using System;
 using System.Collections;
@@ -305,8 +306,13 @@ namespace SqlSugar.MongoDb
             foreach (var group in grouped)
             {
                 BsonDocument doc = new BsonDocument();
+                var existsId = false;
                 foreach (var col in group)
                 {
+                    if (col.DbColumnName == "_id") 
+                    {
+                        existsId = true;
+                    }
                     // 自动推断类型，如 string、int、bool、DateTime、ObjectId 等
                     if (col.IsJson == true)
                     {
@@ -330,7 +336,10 @@ namespace SqlSugar.MongoDb
                         doc[col.DbColumnName] = UtilMethods.MyCreate(col.Value);
                     }
                 }
-
+                if (existsId == false) 
+                {
+                    doc["_id"] = ObjectId.GenerateNewId();
+                }
                 // 转为 JSON 字符串（标准 MongoDB shell 格式）
                 string json = doc.ToJson(UtilMethods.GetJsonWriterSettings());