Get “data from collection b not in collection a” in a MongoDB shell query

Question

I have two MongoDB collections that share a common _id. Using the mongo shell, I want to find all documents in one collection that do not have a matching _id in the other co

Answer 1

I've made a script, marking all documents on the second collection that appears in first collection. Then processed the second collection documents.

var first = db.firstCollection.aggregate([ {'$unwind':'$secondCollectionField'} ])

while (first.hasNext()){ var doc = first.next(); db.secondCollection.update( {_id:doc.secondCollectionField} ,{$set:{firstCollectionField:doc._id}} ); }

...process the second collection that has no mark

db.secondCollection.find({"firstCollectionField":{$exists:false}})