Retrieve Data

Overview

You can use read operations to retrieve data from your MongoDB database. There are multiple types of read operations that access the data in different ways. If you want to request results based on a set of criteria from the existing set of data, you can use a find operation such as the find() or findOne() methods.

You can also further specify the information you are requesting by including additional parameters or by chaining other methods such as:

• Sort Results

• Skip Returned Results

• Limit the Number of Returned Results

• Specify Which Fields to Return

You can also use an aggregation operation to retrieve data. This type of operation allows you to apply an ordered pipeline of transformations to the matched data.

If you want to monitor the database for incoming data that matches a set of criteria, you can use the watch operation to be notified in real-time when matching data is inserted.

Find

The find() method is called on the Collection object that references the collection you want to query. The method accepts a query document that describes the documents you want to retrieve. For more information on how to specify your query document, see our guide on how to Specify a Query.

If you resolve the Promise returned by find(), you receive a reference to a Cursor with which you can navigate matched documents. If you resolve the Promise returned by findOne(), you receive the matching document or null if there are no matches.

    const findResult = await orders.find({
      name: "Lemony Snicket",
      date: {
        $gte: new Date(new Date().setHours(00, 00, 00)),
        $lt: new Date(new Date().setHours(23, 59, 59)),
      },
    });

await cursor.forEach(console.dir);

[
  { name: "Lemony Snicket", type: "horseradish pizza", qty: 1, status: "delivered", date: ... },
  { name: "Lemony Snicket", type: "coal-fired oven pizza", qty: 3, status: "canceled", date: ...},
  ...
]

See the find() and findOne() for fully-runnable examples.

Aggregate

If you want to run a custom processing pipeline to retrieve data from your database, you can use the aggregate() method. This method accepts aggregation expressions to run in sequence. These expressions let you filter, group, and arrange the result data from a collection.

    const aggregateResult = await orders.aggregate([
      {
        $match: {
          date: {
            $gte: new Date(new Date().getTime() - 1000 * 3600 * 24 * 7),
            $lt: new Date(),
          },
        },
      },
      {
        $group: {
          _id: "$status",
          count: {
            $sum: 1,
          },
        },
      },
    ]);

await cursor.forEach(console.dir);

[
  { _id: 'delivering', count: 5 },
  { _id: 'delivered', count: 37 },
  { _id: 'created', count: 9 }
]

See the MongoDB server manual pages on aggregation for more information on how to construct an aggregation pipeline.

Watch / Subscribe

You can use the watch() method to monitor a collection for changes to a collection that match certain criteria. These changes include inserted, updated, replaced, and deleted documents. You can pass this method a pipeline of aggregation comands that sequentially runs on the changed data whenever write operations are executed on the collection.

    const changeStream = orders.watch([
      { $match: { operationType: "insert" } },
      {
        $project: {
          "fullDocument.name": 1,
          "fullDocument.address": 1,
        },
      },
    ]);
    changeStream.on("change", change => {
      const { name, address } = change.fullDocument;
      console.log(`New order for ${name} at ${address}.`);
    });

For a runnable example of the watch() method using the NodeJS driver, see the change streams usage example.