MongoDB Lookup Issue In collection A, we have

"batches" : [
    ObjectId("56fccffbe7c9f9cf7634dbfe")
],


and in Collection B, we have

{ "_id" : ObjectId("56fccffbe7c9f9cf7634dbfe") }


Can we perform a lookup between these 2 collections?
Yes, you can use the $lookup operator as part of an aggregation, after you $unwind the batches array. For example:

db.collectionA.aggregate[

  { $unwind: "$batches" },

  { $lookup:
    from: "collectionB",
    localField: "batches",
    foreignField: "_id",
    as: "collectionB"
  }
]