要获取用户的所有订阅（包括项目），应该是：

firebase.collection('customers').doc(userID).collection('subscriptions').get()
  .then(subscriptions => {
    subscriptions.forEach(subscription => {
      // subscription.data() should give you a subscription with its items
      console.log(subscription.data());
    });
  })

如果订阅存在，则查询方式如下，该订阅对于给定客户具有 状态 'active' ：

firebase.collection('customers').doc(userID).collection('subscriptions')
  .where('status', '==', 'active').get()
  .then(activeSubscriptions => {
    // if this is true, the user has no active subscription.
    console.log(activeSubscriptions.empty)
  });

获取订阅的 状态 ：

firebase.collection('customers').doc(userID).collection('subscriptions').get()
    .then(subscriptions => {
        if (subscriptions.empty) {
            console.log('User has no subscriptions')
        } else if (subscriptions.size > 1) {
            console.log('ERROR: User has multiple subscriptions')
        } else {
            subscriptions.forEach(theSubscription => {
                // there is only this subscription
                const data = theSubscription.data();
                console.log(data.status);
            });
        }
    });