Database

db={"products":[{"_id":"5c41aac4d45a731af564c433","recharge_type":["5c00d9cf7dd7a23f08bdae5e"],"name":"25 OC - Product 1","title":"First installation recharge","description":"0.1 credit for first time installation","offer_message":"Hi.. You got 0.1 credits..!!","brand_id":"5c41a8c34272c61a176b7639","buy_rate":20,"profit_margin":80,"image":"test.jpg","varient":[{"_id":"5c41a9f3f8e1e71aa75b4f32","display_name":"testlia","profit_margin":66,"max_retail_price":66,"sell_rate":66,"sell_rate_local":66,"country_id":"5c01149d3c20440a6b2e4928"},{"_id":"5c4ee53bbc20b27948b3aa99","display_name":"testrinu","profit_margin":66,"max_retail_price":66,"sell_rate":66,"sell_rate_local":66,"country_id":"5c00e1697dd7a23f08bdae68"}],"createdAt":"2019-01-18T10:30:28.991Z","updatedAt":"2019-01-28T11:19:23.662Z","__v":0}],"orders":[{"_id":"5c77a025d65a892f6acf1803","shop_id":"5c7d194ca10ea45c0c03a0ee","brand_id":"5c41a8b44272c61a176b7638","product_varient_id":"5c41a9f3f8e1e71aa75b4f32","buy_rate":10,"buy_rate_after_discount":20,"product_mrp":30,"sell_rate":40,"customer_mobile":123456789,"status":true,"transaction_id":"5c6670c9b6c63d0762c6cc76","product_id":"5c41aac4d45a731af564c433","createdAt":"2019-02-28T08:47:33.097Z","updatedAt":"2019-02-28T08:47:33.097Z","__v":0}]}

Query

db.orders.aggregate([{$lookup:{from:"products",let:{"productId":"$product_id","productVarientId":"$product_varient_id"},pipeline:[{$match:{$expr:{$eq:["$_id","$$productId"]}}},{$addFields:{varient:{$filter:{input:"$varient",as:"varient",cond:{$eq:["$$productVarientId","$$varient._id"]}}}}}],as:"product_data"}}])

Result