根据父级属性查询Mongodb集合

Question

I've got a Posts document that belong to Users, and Users have an :approved attribute. How can I query my Posts using Mongodb st I only get those for where User has :approved => true ?

I could write a loop that creates a new array, but that seems inefficient.

Answer 1

MongoDB does not have any notion of joins.

You've stated in the comments that Posts and Users are separate collections, but your query clearly involves data from both collections, which would imply a join.

I could write a loop that creates a new array, but that seems inefficient.

A join operation in SQL is basically a loop that happens on the server. With no join support on the server side, you'll have to make your own.

Note that many of the libraries (like Morphia) actually have some of this functionality built-in. You are using Mongoid which may have some of this support, but you'll have to do some hunting.

Answer 2

The easiest way to think about it would be to query for unique user ids of users who are approved and then query for post documents where the poster's user_id is in that set.

As Rubish said, you could de-normalize by adding an approved field to the post document. When a user's approval status is toggled (they become approved or unapproved) do an update on the posts collection where, for all of that user's posts, you toggle the denormalized approval field.

Using the denormalized method lets you do one query instead of two (simplifying the logic for the most common case) and isn't too much of a pain to maintain.

Let me know if that makes sense.