使用php将新内部对象插入现有对象mongodb

Question

我的mongo系列中有以下文档

{"_id":"5a9e97557cf28c1c2d00003d",
"user_id":"avi12",
"name":"Avinash",
"Friends":{
"avi12":{
"From":"avi12",
"To":"chandu",
"Friend_status":"pending",
"Time":1520342869,
"requestid":"s_avi12_first"
},
"second_user":{
"From":"avi122",
"To":"chandu2",
"Friend_status":"pending2",
"Time":1520342869,
"requestid":"2s_avi12_first"
}
}
}

我想在Friends对象中插入新对象（“第三用户”）。我试过以下代码......

$update_friend_send=array(
                        $user_name=>array(
                            "From"=>$user_name,
                            "To"=>$To,
                            "Friend_status"=>"pending",
                            "Time"=>time(),
                            "requestid"=>"s_".$user_name."_".$sender_id
                            ));


                    $condition = array("_id"=>$realmongoid);
                    $data = array('$set' => array('Friends.$' =>$update_friend_send));

                    $collection->update($condition,$data);

但是当我运行这段代码时，它通过插入新的（第三个用户）对象并删除旧对象（avi12和第二个）来更新Friends对象，只有最后一个是重命名。

如何将新对象插入到保留以前数据的Friends对象中，我也不想使用数组。请帮忙...