db={"customers":[{customer_id:1,name:"Jim Smith",email:"jim.smith@example.com"},{customer_id:2,name:"Bob Jones",email:"bob.jones@example.com"}],"orders":[{order_id:1,customer_id:1,items:[{order_item_id:1,name:"Foo",price:4.99},{order_item_id:2,name:"Bar",price:17.99}]},{order_id:2,customer_id:1,items:[{order_item_id:3,name:"baz",price:24.99,order_id:2}]}]}

db.customers.aggregate([{$lookup:{from:"orders",localField:"customer_id",foreignField:"customer_id",as:"orders"}}])