Pause and anticipate a request in MongoDB

I am facing an issue with my Typescript async method used to query a MongoDB database using the nodejs driver. The compiler seems to be indicating that the "await" before "myConnectedClient" has no effect on the type of this expression. This has left me confused - is the call to the aggregate() method asynchronous? Do I need to wait for it or not?
Thank you.

async findQuery<T>(
    collection: string,
    findParams: Query<T>,
    sort: Sort<T>,
    myConnectedClient: MongoClient
  ) {
    const firstResult = await myConnectedClient // the compiler indicates await is useless
      .db("ZZZ_TEST_ALL")
      .collection("my_collection_01")
      .aggregate<string>([{ $project: { _id: 0, name: 1 } }]);
    firstResult.forEach((field) => {
      console.log(`Field: ${field}`);
    });
  }

UPDATE: I have discovered that I need to add .toArray() after calling the .aggregate() method; but why exactly is this required? Can someone please explain the underlying mechanism to me? Does aggregate() not have a callback and does it not return a promise? Are there any alternatives to using .toArray()? Thank you.

// now await is functioning correctly
const firstResult = await myConnectedClient
      .db("ZZZ_TEST_ALL")
      .collection("my_collection_01")
      .aggregate<string>([{ $project: { _id: 0, name: 1 } }]).toArray();

Answer №1

If you need to use the Aggregate function in MongoDB with Node.js, keep in mind that it is synchronous and will return an AggregationCursor object.

The AggregationCursor has several asynchronous methods to retrieve the actual data such as toArray, forEach, or a simple iterator.

When working with the cursor in the first code snippet, 'firstResult' represents the cursor itself, so there is no wait needed. However, when using forEach to log the results, make sure you properly handle the promise chain by returning it like so:

const firstResult = myConnectedClient.......; 
return firstResult.forEach(......);

This ensures that the promise returned from findQuery will only be resolved once forEach completes iterating through the results.

In the second code snippet related to "update", if 'firstResult' refers to the data and not the cursor, then await is necessary to get it from toArray(). To achieve this without the explicit cursor, you can do:

const cursor = myConnectedClient.......; 
const firstResult = await cursor.toArray();

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