How to concatenate arrays from multiple documents in mongodb?

@gilbert 

In MongoDB, you can use the $concatArrays aggregation operator to concatenate arrays from multiple documents. Here's an example of how you can do this:

1. Use the $group stage in the aggregation pipeline to group the documents based on a common field.
2. Use the $push operator in the $group stage to create an array of the field you want to concatenate.
3. Use the $project stage in the aggregation pipeline to apply the $concatArrays operator to concatenate the arrays.

Here's an example using the MongoDB shell:

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db.collection.aggregate([
  {
    $group: {
      _id: null,
      fieldToConcatenate: { $push: "$fieldToConcatenate" }
    }
  },
  {
    $project: {
      concatenatedArray: { $concatArrays: "$fieldToConcatenate" }
    }
  }
])

This will concatenate the arrays from multiple documents into a single array and return the result in the "concatenatedArray" field.