Bi-directional Many-to-many relationships using virtuals

Hi guys,

Thanks for adding the virtual population, it’s saved a lot of time! I still have an issue with the many-to-many. I want to be able to use “classics relationships” and virtuals as well.

// Populate
const User = new Schema({
  name: String,
  articles: [{ type: Schema.Types.ObjectId, ref: 'Article' }]
});

const Article = new Schema({
  title: String,
  authors: [{ type: Schema.Types.ObjectId, ref: 'User' }]
});

const article = new Articles({ title: 'My post', authors: ['1234', '5678'] }); 
article.save((err, article) => {...});

Article.find({}).populate('authors') .exec((err, articles) => {...}); // Work fine, with populated values

User.find({}).populate('articles') .exec((err, articles) => {...});  // Normal behavior, the articles attributes is an empty array.

With the new virtual population, I was trying to make something to not have to maintain the consistency between my many-to-many relationships. I would be able to create an article or user and spent some user’s ID or article’s ID when I create them and didn’t have to think about the relationships direction. I tried this below but it does not work.

// Populate + virtuals
const User = new Schema({
  name: String,
  articles: [{ type: Schema.Types.ObjectId, ref: 'Article' }]
});

const Article = new Schema({
  title: String,
  authors: [{ type: Schema.Types.ObjectId, ref: 'User' }]
});

// Try to also apply virtual on existing relationships attributes
User.virtual('articles', {
  ref: 'Article',
  localField: '_id',
  foreignField: 'authors'
});

Article.virtual('authors', {
  ref: 'User',
  localField: '_id',
  foreignField: 'articles'
});

const user1 = new User({ name: 'John Doe' }) // _id = 1234
const user2 = new User({ name: 'Mickey Mouse' }) // _id = 5678 

const article = new Articles({ title: 'My post', authors: ['1234', '5678'] }); 
article.save((err, article) => {...});

Article.find({}).populate('authors') .exec((err, articles) => {...}); // Work fine, with populated values

User.find({}).populate('articles') .exec((err, articles) => {...}); // I would also get the articles thanks to the virtual population

Any solution to do that?

Thanks 😃

[vkarpov15]

Hmm I’d imagine the issue is due to the fact that you have a virtual field with the same name as a real field. That’s not a good idea. If you want to model this sort of many-to-many, I’d recommend having putting the array on the smaller side of the many-to-many and using a virtual for the other side:

// Populate + virtuals
const User = new Schema({
  name: String
});

// Since an article will likely not have hundreds of authors but an author may have hundreds of articles
const Article = new Schema({
  title: String,
  authors: [{ type: Schema.Types.ObjectId, ref: 'User' }]
});

// Try to also apply virtual on existing relationships attributes
User.virtual('articles', {
  ref: 'Article',
  localField: '_id',
  foreignField: 'authors'
});

// Now you should be able to do User.find().populate('articles') and Article.find().populate('authors')

[ckee83]

In the above example is it required to .push authors into the Article collection before it will be returned in a populate?