Retrieve Distinct Values of a Field检索字段的剔除重复值

You can retrieve a list of distinct values for a field across a collection by using the collection.distinct() method. 通过使用collection.distinct()方法，可以检索集合中某个字段的剔除重复值列表。Call the distinct() method on a Collection object with a document field name parameter as a String to produce a list that contains one of each of the different values found in the specified document field as shown below:对Collection对象调用distinct()方法，并将文档字段名参数作为String，以生成一个列表，该列表包含指定文档字段中的每个不同值之一，如下所示：

const distinctValues = myColl.distinct("countries", query);

You can specify a document field within an embedded document using dot notation. 可以使用点表示法在嵌入文档中指定文档字段。If you call distinct() on an document field that contains an array, the method treats each element as a separate value. 如果对包含数组的文档字段调用distinct()，则该方法将每个元素视为一个单独的值。See the following example of a method call to the wins field in the awards subdocument:请参阅以下对awards子文档中的wins字段进行方法调用的示例：

const distinctValues = myColl.distinct("awards.wins", query);

You can specify additional query options using the options object passed as the third parameter to the distinct() method. 您可以使用作为distinct()方法的第三个参数传递的options对象来指定其他查询选项。For details on the query parameters, see the distinct() method in the API documentation.有关查询参数的详细信息，请参阅API文档中的distinct()方法。

If you specify a value for the document field name that is not of type String such as a Document, Array, Number, or null, the method does not execute and returns a TypeMismatch error with a message that resembles the following:如果为文档字段名指定的值不是String类型，例如Document、Array、Number或null，则该方法不会执行，并返回TypeMismatch错误，并显示类似以下消息：

"key" had the wrong type. Expected string, found <non-string type>

Visit Retrieve Distinct Values for more information about the distinct() method.有关Distinct()方法的详细信息，请访问检索剔除重复值。

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 {
    // define a database and collection on which to run the method定义要在其上运行方法的数据库和集合
    const database = client.db("sample_mflix");
    const movies = database.collection("movies");

    // specify the document field指定文档字段
    const fieldName = "year";

    // specify an optional query document指定可选的查询文档
    const query = { directors: "Barbra Streisand" };

    const distinctValues = await movies.distinct(fieldName, query);

    console.log(distinctValues);
  } finally {
    await client.close();
  }
}
run().catch(console.dir);

[ 1983, 1991, 1996 ]