mongodbHow to join two collections in MongoDB?

MongoDB provides the $lookup operator to join two collections. This operator takes two parameters, from and localField. from specifies the collection to join with and localField specifies the field from the current collection to match with the from collection.

Example

db.collection1.aggregate([
    {
        $lookup:
        {
            from: "collection2",
            localField: "field1",
            foreignField: "field2",
            as: "joined_data"
        }
    }
])

Output example

{
    "_id" : ObjectId("5f3d7f9f8f8d7f3d7f9f8d7"),
    "field1" : "value1",
    "joined_data" : [
        {
            "_id" : ObjectId("5f3d7f9f8f8d7f3d7f9f8d7"),
            "field2" : "value1",
            "other_field" : "value2"
        }
    ]
}

Code explanation

• $lookup: The MongoDB operator used to join two collections.
• from: The collection to join with.
• localField: The field from the current collection to match with the from collection.
• foreignField: The field from the from collection to match with the current collection.
• as: The name of the new array field to add to the documents.

Helpful links

• MongoDB Documentation - $lookup

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