db={customers:[{"_id":ObjectId("584aac38686860d502929b8b"),"name":"user","email":"user@gmail.com"}],posts:[{"_id":ObjectId("584aaca6686860d502929b8d"),"title":"Post","description":"description","user_id":"584aac38686860d502929b8b"}]}

db.customers.aggregate([{$lookup:{from:"posts",let:{customerId:"$_id"},pipeline:[{$match:{$expr:{$eq:[{"$toObjectId":"$user_id"},"$$customerId"]}}}],as:"posts"}}])