Firebase-每个用户只允许保存15张图像和15个位置

Question

这是我的Firebase结构。

 { 
   "Users" : {
    "7CeF7ujksLb5t12vDD4vgtPIKGn1" : {
      "images" : {
        "url: 14:36:31 2018-12-15" :    "https://firebasestorage.googleapis.com/v0/b/anthiefy.appspot.com/o/7CeF7ujksLb 
 5t12vDD4vgtPIKGn1%2FImages%2FImg1.jpg?alt=media&token=32c72175-75fa-4607-b894-e1f4bcc08eb0",
        "url: 14:36:48 2018-12-15" : "https://firebasestorage.googleapis.com/v0/b/anthiefy.appspot.com/o/7CeF7ujksLb5t12vDD4vgtPIKGn1%2FImages%2FImg2.jpg?alt=media&token=5e1c5359-2280-4155-8404-926566a73997",
        "url: 14:36:52 2018-12-15" : "https://firebasestorage.googleapis.com/v0/b/anthiefy.appspot.com/o/7CeF7ujksLb5t12vDD4vgtPIKGn1%2FImages%2FImg3.jpg?alt=media&token=d0b48a93-1b17-4789-9090-9e4226c7d1a3"
      },
      "locations" : {
        "location: 14:36:28 2018-12-15" : "14.2684988,-89.7267805",
        "location: 14:36:44 2018-12-15" : "14.2684988,-89.7267805",
        "location: 14:36:49 2018-12-15" : "14.2684988,-89.7267805"

当它们达到极限（15）时，我该怎么办，覆盖前几个？