java - 从 Cloud Firestore 检索复杂文档

Question

我正在尝试从 Cloud Firestore 中检索一组复杂的对象。

数据存储如下:

书籍文档有作者和译者子集合。为了检索完整的图书列表，按如下方式调用数据库(我省略了 Book 对象的所有其他属性，并保留了理解该问题所需的内容):

public void getBooks(final BookRetrievable bookRetrievable) {
    FirebaseFirestore firebaseFirestore = FirebaseFirestore.getInstance();
    ArrayList<Book> books = new ArrayList<>();

    firebaseFirestore.collection("books")
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (DocumentSnapshot documentSnapshot : task.getResult()) {
                            Book book = new Book();
                            book.setId(documentSnapshot.getString("id"));
                            book.setName(documentSnapshot.getString("name"));
                            book.setStatus(Status.valueOf(documentSnapshot.getString("status")));
                            documentSnapshot.getReference().collection("authors")
                                    .get()
                                    .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                                        @Override
                                        public void onComplete(@NonNull Task<QuerySnapshot> task) {
                                            if (task.isSuccessful()) {
                                                ArrayList<Author> authors = new ArrayList<>();
                                                for (DocumentSnapshot documentSnapshot : task.getResult()) {
                                                    Author author = new Author();
                                                    author.setId(documentSnapshot.getString("id"));
                                                    author.setNameInEnglish(documentSnapshot.getString("nameInEnglish"));
                                                    author.setNameInSinhalese(documentSnapshot.getString("nameInSinhalese"));
                                                    author.setWebsite(documentSnapshot.getString("website"));
                                                    authors.add(author);
                                                }

                                                book.setAuthors(authors);

                                                documentSnapshot.getReference().collection("translators")
                                                        .get()
                                                        .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                                                            @Override
                                                            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                                                                if (task.isSuccessful()) {
                                                                    ArrayList<Translator> translators = new ArrayList<>();
                                                                    for (DocumentSnapshot documentSnapshot : task.getResult()) {
                                                                        Translator translator = new Translator();
                                                                        translator.setId(documentSnapshot.getString("id"));
                                                                        translator.setNameInEnglish(documentSnapshot.getString("nameInEnglish"));
                                                                        translator.setNameInSinhalese(documentSnapshot.getString("nameInSinhalese"));
                                                                        translator.setWebsite(documentSnapshot.getString("website"));
                                                                        translators.add(translator);
                                                                    }

                                                                    book.setTranslators(translators);

                                                                    books.add(book);

                                                                    books.sort((Book b1, Book b2) -> b1.getSeriesNo().compareTo(b2.getSeriesNo()));
                                                                    bookRetrievable.onCallback(books);
                                                                } else {
                                                                    bookRetrievable.onCallback(null);
                                                                }
                                                            }
                                                        });
                                            } else {
                                                bookRetrievable.onCallback(null);
                                            }
                                        }
                                    });
                        }
                    } else {
                        bookRetrievable.onCallback(null);
                    }
                }
            });
}

但是，如果我放置日志，似乎有与检索到的书籍数量一样多的回调。因此，有没有一种方法可以优化此代码，使每本书和每个作者和译者文档只有一个回调？

提前致谢!

Answer 1

是的，有。为了得到所有的书对象，不需要创建一个Book类的对象，你可以从DocumentSnapshot对象中得到它，像这样:

FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
CollectionReference booksRef = rootRef.collection("books");
booksRef.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            for (DocumentSnapshot document : task.getResult()) {
                Book book = document.toObject(Book.class);
                //Do whay you need to do with your Book object
            }
        }
    }
});

如果您需要获取特定书籍的作者和译者，请在您的Book 类中添加另外两个字段，author 和translator 和相应的公共(public) setter/getter 。要从图书对象中获取翻译器，您可以使用以下代码行:

Translator translator = book.getTranslator();

以与作者相同的方式。

Author autothor = book.getAuthor();