How to do inner joining in mongodb?

@ryleigh 

In MongoDB, you can perform an inner join operation using the $lookup aggregation stage in the aggregation pipeline. This stage allows you to combine documents from two collections based on a common field.

Here's an example of how to perform an inner join in MongoDB:

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db.collection.aggregate([
  {
    $lookup: {
      from: "secondCollection",
      localField: "commonField",
      foreignField: "commonField",
      as: "joinedData"
    }
  },
  {
    $unwind: "$joinedData"
  },
  {
    $project: {
      // specify the fields you want to include in the final result
      "field1": 1,
      "field2": 1,
      "joinedData.fieldA": 1,
      "joinedData.fieldB": 1
    }
  }
])

In the above example:

• "collection" is the name of the first collection you want to join.
• "secondCollection" is the name of the second collection you want to join.
• "commonField" is the field that is common between the two collections and will be used for the join operation.
• "field1" and "field2" are fields from the first collection.
• "fieldA" and "fieldB" are fields from the second collection.

The $lookup stage performs the join operation, and the $project stage allows you to shape the final output by including or excluding specific fields.

Keep in mind that inner joins in MongoDB are not as straightforward as in relational databases. You need to use the aggregation framework and carefully craft the pipeline stages to achieve the desired result.