$Lookup Operator In Mongodb

db.collection.aggregate([
   {
      $lookup:
         {
           from: <collection to join>,
           localField: <field from the input documents>,
           foreignField: <field from the documents of the "from" collection>,
           as: <output array field>
         }
   }
])

db.Articles.aggregate([
  {
    $lookup: {
      from: "ArticlesComments",
      localField: "_id",
      foreignField: "_subjectid",
      as: "ArticleDetails"
    }
  }
]);

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