How to get multiple document using array of MongoDb id?

MongoDB is type sensitive, which means 1 is different with '1', so are "55880c251df42d0466919268" and ObjectId("55880c251df42d0466919268"). The later one is in ObjectID type but not str, and also is the default _id type of MongoDB document.

You can find more information about ObjectID here.

Just try:

db.getCollection('feed').find({"_id" : {"$in" : [ObjectId("55880c251df42d0466919268"), ObjectId("55bf528e69b70ae79be35006")]}});