How to join two collection in mongodb?

@rollin 

In MongoDB, you can join two collections by using the aggregation framework and the $lookup operator. Here is an example of how you can join two collections:

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db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "field_in_collection1",
      foreignField: "field_in_collection2",
      as: "joined_data"
    }
  }
])

In this example, replace "collection1" and "collection2" with the names of your collections. Also, replace "field_in_collection1" and "field_in_collection2" with the fields that you want to use to join the two collections.

The $lookup operator will match the documents in collection1 with the documents in collection2 based on the specified fields and add the matching documents as an array to the "joined_data" field in the output.