按月Firestore过滤器数据

I'm trying to filter a data collection by month. But it's giving me an error TypeError: _this._firestore.collection('Employee').orderByChild is not a function. Below is my code and database structure

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const month = 5
firestore.collection('Employee').orderByChild("month").equalTo(month).once('value',snapshot=>{
                snapshot.forEach((childSnapshot)=>{
                var users = childSnapshot.key;
                })
            })