mongodb select using association - mongodb

Question

I have two collections, user_logs and users, user_logs documents have user_id field so I need some data from user_logs but in the same query I want to check if some other field from user related to the current user_log is empty. How should I do this?

Answer 1

A query can only access one collection at a time. Mongodb doesn't support joins.
They that's why they recommend that you embed the referenced data inside the document.
If the logs documents for each user isn't too big, then you can change the embed that info inside the user collection.
Giving you something like this.
Embedded User Collection:
{
user_id : "uid1",
logs : [
{ message : "Error: System shutdown", date : "2014-11-11" },
{ message : "Error: System shutdown", date : "2014-11-13" }
]
}
However, if you want to keep your current structure then you're going to have to perform two queries to find related info between the users and user_logs collections.
Example
db.user_logs.insert([
{ _id : "ul1", log : "code 1", user_id : "u1" },
{ _id : "ul2", log : "code 2", user_id : "u1" }
]);
db.users.insert([
{ _id : "u1", name : "bob", user_logs_id : "ul1" },
{ _id : "u2", name : "smith", user_logs_id : "ul2" }
]);
var userId = db.user_logs.findOne({}).user_id;
db.users.findOne({ _id : userId })
//outputs
{ "_id" : "u1", "name" : "bob", "user_logs_id" : "ul1" }