How to merge JSON objects based on key/value pair?

I need to combine results from two different documents in Mongo. I have a function like this:

async function getReviewsByUserId (req, res) {
  const { userId } = req.params

  const reviews = await Review.find({ userId }).lean() || []

  return res.status(200).send(reviews.reverse())
}

The reviews array looks like this:

{
    "_id" : ObjectId("1263b55ef2cdd3ebb0654d1dd"),
    "launchId" : "7fb83b40-7c6f-4099-aaed-fe9d0dc03111",
    "userId" : "1",
}
{
    "_id" : ObjectId("6355565cf5ef2cddebb065584"),
    "launchId" : "12b53940-136f-3399-aaed-fe9d0dc05473",
    "userId" : "7fb83b40-7c6f-4099-aaed-fe9d0dc03112",
}

I need to use the launchId from each review, look up a launch object from my mongo database, and combine that with the correct object in the reviews array.

Example of what I mean:

async function getReviewsByUserId (req, res) {
  const { userId } = req.params

  const reviews = await Review.find({ userId }).lean() || []

  const launches = await Launch.find(/* find all launches where launch._id is equal to reviews.launchId*/)

  return res.status(200).send(launches.reverse())
}

So if launches data looks like this (and launches is also an array of ALL launches):

{
    "_id" : "12b53940-136f-3399-aaed-fe9d0dc05473",
    "name" : "The Park",
}

Then how can I merge this with the reviews payload where the launch._id == reviews.launchId so that the final data looks like this:

{
    "_id" : ObjectId("1263b55ef2cdd3ebb0654d1dd"),
    "launchId" : "7fb83b40-7c6f-4099-aaed-fe9d0dc03111",
    "userId" : "1",
}
{
    "_id" : ObjectId("6355565cf5ef2cddebb065584"),
    "launchId" : "12b53940-136f-3399-aaed-fe9d0dc05473",
    "userId" : "7fb83b40-7c6f-4099-aaed-fe9d0dc03112",
    "launch": {
        "_id" : "12b53940-136f-3399-aaed-fe9d0dc05473",
        "name" : "The Park",
    }
}

>Solution :

This could be achieved by using aggregate pipelines.

1. Filter reviews by userId in $match stage
2. $lookup for launches
3. $unwind the launch array to an object

The solution could be:

  async function getReviewsByUserId(req, res) {
    const { userId } = req.params;

    const launches = await Review.aggregate([
      {
        $match: {
          userId
        }
      },
      {
        $lookup: {
          from: "launches",
          localField: "launchId",
          foreignField: "_id",
          as: "launch"
        }
      },
      {
        $unwind: {
          path: "$launch",
          preserveNullAndEmptyArrays: true
        }
      }
    ]);

    return res.status(200).send(launches.reverse());
  }