Uncertainty about usage of populate()

[f0o]

Hi!

I'm used to ORMs with the usual User->HasMany("Post") mentality in which each Post would have an entry with user_id that points to the User's ID.

Coming from those traditional relational concepts I'm now thrown off by mongoose' populate..

It seems to me that using the same example I would have Posts: [{type: Schema.Types.ObjectId, ref: "Post" }] in the User model and User: {type: Schema.Types.ObjectId, ref: "User"} in the Post model...

But now if User makes a new Post and sets Post.User=MyUser._id it seems like I would also need to update the User object and append the newly created Post._id for the populate to work...
Is that correct? Am I missing something?

It seems a bit weird to me that I need to update/amend the User record every time a new Post is being created since the lookup glue already exists in the Post schema... It feels counter intuitive coming from traditional relational databases...

I just want to be sure this is the correct way and I'm not mixing things up or making it unnecessarily harder

[vkarpov15]

@f0o that is correct, you'd need to update Post.user and User.posts. Mongoose doesn't do that for you because the lookup glue is in Mongoose, not MongoDB.

A better alternative would be to store the reference in one place, ideally just User: {type: Schema.Types.ObjectId, ref: "User"}, and make Posts a virtual: https://mongoosejs.com/docs/tutorials/virtuals.html#populate

[f0o]

Virtual makes a lot of sense, it feels a lot like those traditional Has* relationship glues - I think that's exactly what I was looking for!

Thanks @vkarpov15 !

[f0o]

@vkarpov15 I may have a followup question if that's alright...

I was naively looking at how to get the newest Posts based on the createdAt attribute so I whipped up:

User.virtual('posts.new', {
  ref: 'Post',
  localField: '_id',
  foreignField: 'user_id',
  match: {
    "createdAt": {
      "$gte": () => { return new Date(new Date().setDate(new Date().getDate()-2))}
    }
  }
})

which obviously fails because Function was not expected there and instead a Date object was expected...

Is there a way to create a sliding-window typed relation inside the Schema?
I cant just use the new Date() call directly as it freezes the value in time and on the next find().populate() call it would yield the old results instead of the potential new ones (granted in this specific case the new ones are a subset of the old ones, but that's not the point here)

//EDIT:

I found a solution by abusing the Moment.js support:

  match: {
    "createdAt": {
      "$gte": { valueOf: () => { return new Date(new Date().setDate(new Date().getDate()-2)) } }
    }
  }

[vkarpov15]

The valueOf() hack works. Another approach is to make match a function: http://thecodebarbarian.com/mongoose-5-5-static-hooks-and-populate-match-functions.html#populate-match-function, or pass match in your populate() call.

[f0o]

Ah so the whole match block is the function and not just the element - gotcha!

Gonna do that instead of the valueOf() hack ;)