无法在Firebase实时数据库中发布对象

Question

      let selectedFile;
    let  file = document.getElementById('file');
    file.addEventListener('change', function(event){
        selectedFile = event.target.files[0];
    });

    function uploadFile(){
        let fileName = selectedFile.name;
        let storageRef = firebase.storage().ref('/dogImages/'+fileName);
        let uploadTask = storageRef.put(selectedFile); // uploads file to firebase storge
        var postData = {};


        uploadTask.on('state_changed', function(snapshot){
        },

        function(error){
            console.log(error.message);
        },

        function(){ //handle successful uploads


            uploadTask.then((snapshot) => {
                snapshot.ref.getDownloadURL().then((url) => {
                    postData['url'] = url; // get url of image
                });

            });


            // updates['/users/'+postKey] = postData;

            // firebase.database().ref().update(updates);

            var postKey = firebase.database().ref('users/').push().key;
            firebase.database().ref().child('users/'+postKey).set(postData);
            firebase.database().ref().child('users/'+postKey).set("have a nice day");

  });

我正在做的是将图像保存在Firebase存储器上，然后将其链接保存在Firebase数据库上。一个字符串值已成功添加到数据库，而我无法保存该对象。