Firestore：如何在 forEach() 循环中运行批量写入语句？

Question

我正在学习 Firestore 的 batch writes 方法，它看起来非常简洁。几乎是异步的！但是，我需要一些帮助来弄清楚在查询上执行 forEach() 时如何运行批处理语句。

我的用例是，当用户删除帖子时，我还需要“清理”并更新/删除与该帖子关联的其他项目。这可能是用户为此帖子创建的所有书签、点赞等。

这是deletePost 函数的示例。 如何对bookmarksQuery 和usersAnswerQuery 查询运行批处理语句？

async deletePost(post) {
  const response = confirm('Delete this post?')
  const batch = this.$fire.firestore.batch()
  if (response === true && this.userProfile.uid === this.post.uid) {
    try {
      const postRef = this.$fire.firestore
        .collection(`users/${post.uid}/posts`)
        .doc(this.post.id)

      const answerRef = this.$fire.firestore
        .collection('answers')
        .doc(this.post.commentIdWithAnswer)

      const usersAnswerQuery = await this.$fire.firestore
        .collectionGroup('answers')
        .where('id', '==', this.post.commentIdWithAnswer)
        .get()

      const bookmarksQuery = await this.$fire.firestore
        .collectionGroup('bookmarks')
        .where('id', '==', this.post.id)
        .get()

      batch.update(postRef, {
        published: false,
        deleted: true,
        updatedAt: this.$fireModule.firestore.FieldValue.serverTimestamp()
      })

      bookmarksQuery.forEach((doc) => doc.ref.delete()) //<---- how to add this to batch?

      usersAnswerQuery.forEach((doc) => doc.ref.delete()) //<---- how to add this to batch?

      batch.delete(answerRef)

      await batch.commit()
      // To do: delete all user 'likes' associated with this post
      
      
      alert('Post successfully deleted!')
    } catch (error) {
      console.error('error deleting post.', error)
    }
  } else {
    return null
  }
}

Answer 1

要将文档删除添加到批处理中，您可以使用 WriteBatch#delete()，就像对 answerRef 所做的那样：

// prepare the batch
const batch = firebase.firestore().batch();

// add each doc's deletion to the batch
docs.forEach((doc) => batch.delete(doc.ref));

// commit the changes
await batch.commit();

虽然上述方法工作正常，但批量写入具有limit of 500 operations。由于您在整理书签、答案和点赞时可能会达到热门帖子的限制，因此我们需要处理这种情况。我们可以通过跟踪您添加到批次中的操作数量来实现这一点，并在每次达到限制时创建一个新批次。

// prepare the batch
let currentBatch = firebase.firestore().batch();
let currentBatchSize = 0;
const batches = [ currentBatch ];

// add each doc's deletion to the batch
docs.forEach((doc) => {
  // when batch is too large, start a new one
  if (++currentBatchSize >= 500) {
    currentBatch = firebase.firestore.batch();
    batches.push(currentBatch);
    currentBatchSize = 1;
  }

  // add operation to batch
  currentBatch.delete(doc.ref);
})

// commit the changes
await Promise.all(batches.map(batch => batch.commit()));

我在您当前的代码中注意到的其他内容：

• deletePost 具有不一致的返回类型 Promise<void | null> - 考虑返回 Promise<boolean>（表示成功，因为您正在处理函数中的错误）
• 您在检查帖子是否可以被当前用户实际删除之前要求用户确认 - 您应该先检查
• 静默删除其他用户的帖子失败，而不是显示错误（这也应由安全规则强制执行）
• 静默删除帖子失败，不向用户显示消息
• 你有一个大的if 块，后面跟着一个小的else 块，你应该翻转它，这样你就可以"fail-fast" 并且不需要缩进大部分代码。

应用解决方案加上这些其他更改：

async deletePost(post) {
  if (this.userProfile.uid !== this.post.uid) {
    alert("You can't delete another user's post.");
    return false; // denied
  }

  const response = confirm('Delete this post?')
  
  if (!response)
    return false; // cancelled
  
  try {
    const postRef = this.$fire.firestore
      .collection(`users/${post.uid}/posts`)
      .doc(this.post.id)

    const answerRef = this.$fire.firestore
      .collection('answers')
      .doc(this.post.commentIdWithAnswer)

    const usersAnswerQuery = await this.$fire.firestore
      .collectionGroup('answers')
      .where('id', '==', this.post.commentIdWithAnswer)
      .get()

    const bookmarksQuery = await this.$fire.firestore
      .collectionGroup('bookmarks')
      .where('id', '==', this.post.id)
      .get()
      
    let currentBatch = this.$fire.firestore.batch();
    const batches = [currentBatch];    

    currentBatch.update(postRef, {
      published: false,
      deleted: true,
      updatedAt: this.$fireModule.firestore.FieldValue.serverTimestamp()
    });
    
    currentBatch.delete(answerRef);
    
    let currentBatchSize = 2;

    const addDocDeletionToBatch = (doc) => {
      if (++currentBatchSize >= 500) {
        currentBatch = this.$fire.firestore.batch();
        batches.push(currentBatch);
        currentBatchSize = 1;
      }
    
      currentBatch.delete(doc.ref);
    }

    bookmarksQuery.forEach(addDocDeletionToBatch)
    usersAnswerQuery.forEach(addDocDeletionToBatch)

    // TODO: delete all user 'likes' associated with this post

    // commit changes
    await Promise.all(batches.map(batch => batch.commit()));
    
    alert('Post successfully deleted!')
    return true;
  } catch (error) {
    console.error('error deleting post.', error)
    alert('Failed to delete post!');
    return false;
  }
}

注意：如果您使用标准 cmets // TODO 和 // FIXME，您可以使用许多识别和突出显示这些 cmets 的工具。

Answer 2

请按以下步骤操作。不要忘记批量写入（包括删除）的 500 个文档限制。

async deletePost(post) {
  const response = confirm('Delete this post?')
  const batch = this.$fire.firestore.batch()
  if (response === true && this.userProfile.uid === this.post.uid) {
    try {
      // ...

      batch.update(postRef, {
        published: false,
        deleted: true,
        updatedAt: this.$fireModule.firestore.FieldValue.serverTimestamp()
      })

      bookmarksQuery.forEach((doc) => batch.delete(doc.ref))

      usersAnswerQuery.forEach((doc) => batch.delete(doc.ref))

      batch.delete(answerRef)

      await batch.commit()
      // To do: delete all user 'likes' associated with this post
      
      
      alert('Post successfully deleted!')
    } catch (error) {
      console.error('error deleting post.', error)
    }
  } else {
    return null
  }
}