var Book = new Schema({
    ISBN: String,
    title: String,
    author: String,
    image: String,
    availability: [{zipcode: String,
                    total: Number,
                    loaned: Number
    }]
});

db.col.aggregate(
  [
    {$match:{$expr:{$gt:[{"$sum":"$availability.total"}, {"$sum":"$availability.loaned"}]}}},
    { $sample: { size: 25 } }
  ]
)