通过稍微调整你的promises链，它应该可以做到这一点，见下文：

exports.getNewReport = functions.database.ref('/Hotel_Complaints/Users/{usersId}/')
    .onCreate((snapshot, context) => {
        // Grab the current value of what was written to the Realtime Database.
        var user_id = context.params.usersId;
        console.log(user_id);
        // Grab the current value of what was written to the Realtime Database.
        var eventSnapshot = snapshot.val();

        var device_token = admin.database().ref('/Hotel_Staff/' + user_id + '/device_token').once('value');

        return device_token
            .then(result => {

                var token_id = result.val();

                console.log(token_id);

                var str = eventSnapshot.issue_description;

                var payload = {
                    notification: {
                        title: "New complaint",
                        body: "New complaint for your department"
                    }
                };

                // Send a message to devices subscribed to the provided topic.
                return admin.messaging().sendToDevice(token_id, payload);
            })
            .then(response => {
                // See the MessagingTopicResponse reference documentation for the
                // contents of response.
                console.log("Successfully sent message:", response);
                return null;
            })
            .catch(error => {
                console.log("Error sending message:", error);
                return null
            });

    });