MongoDB 文档设计决策

Question

I'm building an API for a small social network and I came across a design decision that I have to make. I'm working with Express and MongoDB with mongoose to deal with the database.

I have two Documents: Users and Posts. I want the Users to be able to mark Posts as their favorites. I came up with two different ways for the Implementation:

Option A: Saving the favorites in the User Document. It makes it easy to show all favorite posts of an user. But how would I query the users, that have favorited a specific Post?

UserSchema:
   favorite_posts: [
      {
         type: mongoose.Schema.Types.ObjectId,
         ref: "posts"
      }
   ]

Option B: Saving the Users, that hit the favorite button in the Post Document. The benefit would be, that you can easily display all Users, that have favorited a Post. But how do I list all Posts that one specific User has marked as favorites.

PostSchema:
   users_favorited: [
      {
         type: mongoose.Schema.Types.ObjectId,
         ref: "users"
      }
   ]

Can somebody explain me how to query such things? I'm not getting smarter from the documentation... :(

Answer 1

As already mentioned in the comments your best bet would be a join-table to make an:m relation work. Mongoose does emulate the sql inner-join functionality through the populate() functionality in regular queries or the $lookup-step in an aggregation. So basically create a table called "likes" that only holds refs to the user and the post. Using the aggregation framework, you can then easily query for all likes of a user or all likes on a post by first using the $match operator, then $group by either the user or the post and $push to create an array of all likes of a user or vice versa and then join the needed data on it using the $lookup step.

However, you could, as you've decribed, put all the favorites in a array on either the user- or the post-documents, but unless you know for sure that these arrays won't grow large, I'd recommend against it, as mongoDb is not designed for this kind of usage and you'll very quickly run into performance problems. See http://www.askasya.com/post/largeembeddedarrays/ for more.

Answer 2

If you are gonna query a lot by userid, you can just add a userid column on the favorites document. This would save queries/joins/aggregations