mongodbHow to do joins in MongoDB?

MongoDB supports various types of joins to combine documents from multiple collections. The most commonly used join is the $lookup operator. It performs a left outer join to an unsharded collection in the same database to filter in documents from the “joined” collection for processing.

db.collection.aggregate([
  {
    $lookup:
      {
        from: "otherCollection",
        localField: "field1",
        foreignField: "field2",
        as: "alias_name"
      }
   }
])

The above example code performs a $lookup operation on the collection collection, joining documents from the otherCollection collection. The localField and foreignField parameters specify the fields used to match documents from the two collections. The as parameter specifies the name of the new array field in the input documents.

Other join operations supported by MongoDB include $merge, $graphLookup, $facet, and $unwind.

Helpful links

• MongoDB Documentation - Joins
• MongoDB Documentation - $lookup
• MongoDB Documentation - $merge
• MongoDB Documentation - $graphLookup
• MongoDB Documentation - $facet
• MongoDB Documentation - $unwind

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