使用Swift和Firebase提取多个数据

Question

如何基于来自另一个节点的数据来获取数据？在下面的示例中，我希望获取所有fatherName = John的folderID，然后对于每个folderID，在Fetch table数组中获取相应的数组。

func fetchData(completion: @escaping ([Fetch])->()) {
    let currentUser = Auth.auth().currentUser!
    let postRef = self.databaseRef.child("father").queryOrdered(byChild: "fatherName").queryEqual(toValue: "John")
    postRef.observe(.value, with: { (snapshot) in

        var fatherArray = [Father]()
        var followArray = [Fetch]()

        for topic in snapshot.children {
            let fatherName = Father(snapshot: topic as? DataSnapshot)
            fatherArray.append(fatherName)

        }
        //for each followerId in fatherArray, retrieve the snapshot (not sure how to do this part)

        completion(followArray)

    }) { (error) in
        print(error.localizedDescription)
    }

}