从Firebase GET计算满足多个条件的对象

I have a project to implement Golang integration with Firebase. The goal sounds pretty simple: I want to count how many objects in database that meet the condition (for this example if the location is in POOL XYZ and time_out is 0) and return it with format like this:

pool XYZ,n

Where poolXYZ is from the Location and n is number(s) of the objects

This is my JSON Firebase structure example (for testing)

"vehicles" : {
    "VB201" : {
      "register" : "VB201",
      "police_number" : "",
      "position" : {
        "content" : {
          "location" : "POOL XYZ",
          "stat" : 0,
          "time_in" : 1555331615429,
          "time_out" : 0,
          "username" : "POOL XYZ1"
        }
      },
      "trips" : {
        "content" : {
          "accept" : "POOL XYZ",
          "stat" : 1,
          "status" : 0,
          "time_stop" : 1555331615429,
          "username" : "POOL XYZ1"
        }
      }
    }
      }
 }

The things that i want to do is get the "register" of the vehicle, VB201, cause it meet the condition (POOL XYZ and time_out 0). There're several objects like this and has same JSON structure. SO the result might be

POOL XYZ, 1