如何从firebase检索贷款信息（贷款文件字段）和特定贷款中的项目（贷款文件的项目集合）？

Question

我正在尝试通过其 ID 和该贷款中项目的详细信息来获取特定贷款的信息，以将其显示在详细信息页面中。但是，我无法也不知道如何使用提供给我的贷款服务的 getLoanById 方法检索贷款和贷款项目。

detail.page.ts

export class DetailPage {
  loan: Loan;
  loanId: string;

  constructor(private route: ActivatedRoute, private loanService: LoanService) {
    this.loanId = this.route.snapshot.params.id;
    this.loan = this.loanService.getLoanById(this.loanId);
  }
}

loan.service.ts

  getLoanById(id: string) {
    return firebase.firestore().collection('loans').doc(id).get().then(doc => {
      let loan = new Loan(doc.data().username, doc.data().status, doc.data().duedate.toDate(), doc.id);

      return firebase.firestore().collection('loans/' + id + '/items').get().then(collection => {
        loan.items = []; // Empty array
        collection.forEach(doc => {
          let item = new Item(doc.id, doc.data().quantity);
          loan.items.push(item);
        })
        return loan;
      });
    });
  }

Answer 1

将getLoanById()改成如下方法：

 async getLoanById(id: string) : Promise<any> {
    const doc = await firebase.firestore().collection('loans').doc(id).get();
    let loan = new Loan(doc.data().username, doc.data().status, doc.data().duedate.toDate(), doc.id);
    const collection = await firebase.firestore().collection('loans/' + id + '/items').get();
    loan.items = []; // Empty array
    collection.forEach(doc_1 => {
      let item = new Item(doc_1.id, doc_1.data().quantity);
      loan.items.push(item);
    });
    return loan;
  }

由于get() 方法是异步的，因此使用async/await 等待直到检索到数据然后将其添加到列表中。然后在DetailPage 中，您可以执行以下操作：

   constructor(private route: ActivatedRoute, private loanService: LoanService) {
    this.loanId = this.route.snapshot.params.id;
    this.loanService.getLoanById(this.loanId).then((result){
      console.log(result);
     });
  }

then() 方法返回一个Promise。它最多需要两个参数：Promise 的成功和失败情况的回调函数。一旦 Promise 被履行或拒绝，相应的处理函数（onFulfilled 或 onRejected）将被异步调用。

检查：

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function