统计文档数量

Node.js 驱动程序为集合中的文档计数提供了两种方法

collection.countDocuments() 返回与指定查询匹配的集合中的文档数量。如果您指定一个空的查询文档，countDocuments() 返回集合中的文档总数。
collection.estimatedDocumentCount() 返回集合中文档数量的一个估计，基于集合元数据。

estimatedDocumentCount() 比 countDocuments() 快，因为估计使用的是集合的元数据，而不是扫描集合。相比之下，countDocuments() 返回时间较长，但提供了文档数量的准确计数，并支持指定过滤器。根据您的负载选择适当的方法。

要指定要计数的文档，countDocuments() 接受一个查询参数。 countDocuments() 计数匹配指定查询的文档。

countDocuments() 和 estimatedDocumentCount() 支持影响方法执行的可选设置。有关更多信息，请参阅每个方法的参考文档。

提示

使用countDocuments()方法返回集合中文档的总数时，可以通过避免集合扫描来提高性能。为此，请使用提示来利用_id字段的内置索引。仅在调用countDocuments()时带有空查询参数时使用此技术。

collection.countDocuments({}, { hint: "_id_" });

示例

以下示例估计了sample_mflix数据库中movies集合中的文档数量，然后使用带有Canada字段中Canada的精确计数返回movies集合中的文档数量。

注意

您可以使用此示例连接到MongoDB实例并交互包含示例数据的数据库。有关连接到MongoDB实例和加载示例数据集的更多信息，请参阅使用示例指南。

1 // Count documents in a collection
2 
3 import { MongoClient } from "mongodb";
4 
5 // Replace the uri string with your MongoDB deployment's connection string
6 const uri = "<connection string uri>";
7 
8 const client = new MongoClient(uri);
9 
10 async function run() {
11   try {
12     const database = client.db("sample_mflix");
13     const movies = database.collection("movies");
14 
15     /* Print the estimate of the number of documents in the
16     "movies" collection */
17     const estimate = await movies.estimatedDocumentCount();
18     console.log(`Estimated number of documents in the movies collection: ${estimate}`);
19 
20     /* Print the number of documents in the "movies" collection that
21     match the specified query */
22     const query = { countries: "Canada" };
23     const countCanada = await movies.countDocuments(query);
24     console.log(`Number of movies from Canada: ${countCanada}`);
25   } finally {
26     // Close the connection after the operations complete
27     await client.close();
28   }
29 }
30 // Run the program and print any thrown exceptions
31 run().catch(console.dir);

1 // Count documents in a collection
2 
3 import { MongoClient } from "mongodb";
4 
5 // Replace the uri string with your MongoDB deployment's connection string
6 const uri = "<connection string uri>";
7 
8 const client = new MongoClient(uri);
9 
10 async function run() {
11   try {
12     const database = client.db("sample_mflix");
13     const movies = database.collection("movies");
14 
15     /* Print the estimate of the number of documents in the
16     "movies" collection */
17     const estimate = await movies.estimatedDocumentCount();
18     console.log(`Estimated number of documents in the movies collection: ${estimate}`);
19 
20     /* Print the number of documents in the "movies" collection that
21     match the specified query */
22     const query = { countries: "Canada" };
23     const countCanada = await movies.countDocuments(query);
24     console.log(`Number of movies from Canada: ${countCanada}`);
25   } finally {
26     // Close the connection after the operations complete
27     await client.close();
28   }
29 }
30 // Run the program and print any thrown exceptions
31 run().catch(console.dir);

注意

相同的代码片段

上面的JavaScript和TypeScript代码片段相同。此用例中与TypeScript相关的驱动程序没有特定功能。

运行前面的示例代码，您将看到以下输出

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

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不同字段值

1	// Count documents in a collection
2
3	import { MongoClient } from "mongodb";
4
5	// Replace the uri string with your MongoDB deployment's connection string
6	const uri = "<connection string uri>";
7
8	const client = new MongoClient(uri);
9
10	async function run() {
11	try {
12	const database = client.db("sample_mflix");
13	const movies = database.collection("movies");
14
15	/* Print the estimate of the number of documents in the
16	"movies" collection */
17	const estimate = await movies.estimatedDocumentCount();
18	console.log(`Estimated number of documents in the movies collection: ${estimate}`);
19
20	/* Print the number of documents in the "movies" collection that
21	match the specified query */
22	const query = { countries: "Canada" };
23	const countCanada = await movies.countDocuments(query);
24	console.log(`Number of movies from Canada: ${countCanada}`);
25	} finally {
26	// Close the connection after the operations complete
27	await client.close();
28	}
29	}
30	// Run the program and print any thrown exceptions
31	run().catch(console.dir);