Count Documents计数文档

Overview概述

In this guide, you can learn how to count the number of documents in your MongoDB collections. The Node.js driver provides two methods for counting documents in a collection:在本指南中，您可以学习如何计算MongoDB集合中的文档数量。Node.js驱动程序提供了两种对集合中的文档进行计数的方法：

• collection.countDocuments() returns the number of documents in the collection that match the specified query. If you specify an empty query document, countDocuments() returns the total number of documents in the collection.返回集合中与指定查询匹配的文档数。如果指定空查询文档，countDocuments()将返回集合中的文档总数。
• collection.estimatedDocumentCount() returns an estimation of the number of documents in the collection based on collection metadata.基于集合元数据返回集合中文档数量的估计值。

estimatedDocumentCount() is faster than countDocuments() because the estimation uses the collection's metadata rather than scanning the collection. 比countDocuments()更快，因为估计使用集合的元数据而不是扫描集合。In contrast, countDocuments() takes longer to return, but provides an accurate count of the number of documents and supports specifying a filter. Choose the appropriate method for your workload.相比之下，countDocuments()返回的时间更长，但提供了文档数量的准确计数，并支持指定筛选器。为您的工作量选择合适的方法。

To specify which documents you wish to count, countDocuments() accepts a query parameter. countDocuments() counts the documents that match the specified query.要指定要计数的文档，countsDocuments()接受一个查询参数。countDocuments()对与指定查询匹配的文档进行计数。

countDocuments() and estimatedDocumentCount() support optional settings that affect the method's execution. Refer to the reference documentation for each method for more information.countsDocuments()和estimatedDocumentCount()支持影响方法执行的可选设置。有关更多信息，请参阅每种方法的参考文档。

collection.countDocuments({}).hint("_id");

countDocuments() Example: Full File示例：完整文件

The following example estimates the number of documents in the movies collection in the sample_mflix database, and then returns an accurate count of the number of documents in the movies collection with Canada in the countries field:以下示例估计sample_mflix数据库中movies集合中的文档数量，然后在countries字段中返回movies集合中加拿大文档数量的准确计数：

// Count documents in a collection清点集合中的文档

import { MongoClient } from "mongodb";

// Replace the uri string with your MongoDB deployment's connection string将uri字符串替换为MongoDB部署的连接字符串
const uri = "<connection string uri>";

const client = new MongoClient(uri);

async function run() {
  try {
    const database = client.db("sample_mflix");
    const movies = database.collection("movies");

    /* Print the estimate of the number of documents in the "movies" collection打印“电影”集合中文档数量的估计值 */
    const estimate = await movies.estimatedDocumentCount();
    console.log(`Estimated number of documents in the movies collection: ${estimate}`);

    /* Print the number of documents in the "movies" collection that match the specified query打印“电影”集合中与指定查询匹配的文档数量 */
    const query = { countries: "Canada" };
    const countCanada = await movies.countDocuments(query);
    console.log(`Number of movies from Canada: ${countCanada}`);
  } finally {
    // Close the connection after the operations complete操作完成后关闭连接
    await client.close();
  }
}
// Run the program and print any thrown exceptions运行程序并打印任何抛出的异常
run().catch(console.dir);

Running the preceding sample code results in the following output:运行前面的示例代码会得到以下输出：

Estimated number of documents in the movies collection: 23541
Number of movies from Canada: 1349

API Documentation文档

To learn more about any of the types or methods discussed in this guide, see the following API documentation:要了解有关本指南中讨论的任何类型或方法的更多信息，请参阅以下API文档：

• MongoClient
• Db
• Collection
• countDocuments()