我可以确定查询是否找不到具有指定范围内的值的任何子项? [英] Can I determine if query does NOT find any children with values within a specified range?
问题描述
我可以通过某种方法确定 ref
中是否存在 -1
和 newTotal
?
这是我希望能够工作的。如果子查询中存在子查询,则回调运行。
ref.orderByValue()。startAt(-1).endAt(newTotal) .limitToLast(1).once(child_added,function(snap){
/ *只在查询产生结果时运行* /
if(snap.exists( )){
...
} else {
...
}
});
newTotal
是0或任何正整数。 为了检测什么时候没有孩子,你需要(也)监听的值
。事件:
$ p $ var query = ref.orderByValue()。startAt(-1).endAt(newTotal).limitToLast(1)
query.once(value,function(snapshot){
if(snapshot.exists()){
snapshot.forEach(function(child){
});
} else {
console.log('No child exists');
}
});
Can I somehow determine if no children at ref
exist with vals between -1
and newTotal
?
Here's what I was hoping would work. The callback runs if children exist within the query.
ref.orderByValue().startAt(-1).endAt(newTotal).limitToLast(1).once("child_added", function(snap){
/* Runs only when query yields a result */
if (snap.exists()){
...
} else {
...
}
});
newTotal
is 0 or any positive integer.
The child_
events will only fire when the relevant child exists/changes.
To detect when there are no children, you'll need to (also) listen for the value
event:
var query = ref.orderByValue().startAt(-1).endAt(newTotal).limitToLast(1)
query.once("value", function(snapshot){
if (snapshot.exists()){
snapshot.forEach(function(child) {
..
});
} else {
console.log('No child exists');
}
});
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