Ytukyg

I am working on a web application that uses a mongoDB database and express/nodeJS. I want to create a project in which I have users, and users can have posts, which can have many attributes, such as title, creator, and date. I am confused how to do this so that I avoid replication in my database. I tried references by using ids in a list of all the users posts like this idea: [postID1, postID2, postID3, etc...]. The problem is that I want to be able to use query back to all the users posts and display them in an ejs template, but I don't know how to do that. How would I use references? What should I do to make a this modeling system optimal for relationships?

Any help would be greatly appreciated!

Thank you!

This is a classic parent-child relationship, and your problem is that you're storing the relationship in the wrong record :-). The parent should never contain the reference to the children. Instead, each child should have a reference to the parent. Why not the other way around? It's a bit of a historical quirk: it's done that way because a classic relational table can't have multiple values for a single field, which means you can't store multiple child IDs easily in a relational table, whereas since each child will only ever have one parent, it's easy to setup a single field in the child. A Mongo document can have multiple values within a single field by using arrays, but unless you really have a good reason to do so, it's just better to follow the historical paradigm.

How does this apply in your situation? What you're trying to do is to store references to all the children (i.e. the post IDs) as a list in the parent (i.e. an array in the user document). This is not the usual way to do this. Instead, in each child (i.e. in each post), have a field called user_id, and store the userID there.

Next, make sure you create an index on the user_id field.

With that setup, it's easy to take a post and figure out who the user was (just look at the user_id field). And if you want to find all of a user's posts, just do posts.find({user_id: 'XXXX'}). If you have an index on that field, the find will execute quickly.

Storing parent references in the child is almost always better than storing child references in the parent. Even though Mongo is flexible enough to allow you to structure it either way, it's not preferred unless you have a real reason for it.

EDIT
If you do have a valid reason for storing the child references in the parent, then assuming a structure like this:
user = {posts: [postID1, postID2, postID3, ...]}
You can find the user for a specific post by user.find({posts: "XXXX"}). MongoDB is smart enough to know that you're searching for a user in which the post array contains element "XXX". And if you create an index on the posts field, then the query should be pretty quick.