db={"items":[{"item_id":1234,"item_type":"book","item_type_id":1},{"item_id":5678,"item_type":"fruit","item_type_id":1}],"fruits":[{"item_id":1,"title":"Gone with the wind","author":"Margaret Mitchell"}],"books":[{"item_id":1,"name":"Banana","color":"yellow"}]}

db.items.aggregate([{$lookup:{from:"fruits",localField:"item_type_id",foreignField:"item_id",as:"fruits_details"}},{$lookup:{from:"books",localField:"item_type_id",foreignField:"item_id",as:"books_details"}},{$addFields:{books_details:{$cond:[{$eq:["$item_type","book"]},"$books_details",{}]},fruits_details:{$cond:[{$eq:["$item_type","fruit"]},"$fruits_details",{}]}}}])