db={"quotations":[{"_id":ObjectId("5a934e000102030405000002"),"customer":ObjectId("5a934e000102030405000008"),"quote":"To be or not to be"},{"_id":ObjectId("5a934e000102030405000003"),"customer":ObjectId("5a934e000102030405000009"),"quote":"I have a dream"}],"customers":[{"_id":ObjectId("5a934e000102030405000008"),"firstName":"bob","email":"bobsmith@gmail.com","age":50},{"_id":ObjectId("5a934e000102030405000009"),"firstName":"sally","email":"sallyjones@gmail.com","age":60}]}

db.quotations.aggregate([{$lookup:{"from":"customers","localField":"customer","foreignField":"_id","as":"customer"}},{$match:{"customer.firstName":"bob"}},{$unwind:"$customer"}])