Mongoose query populate match id of find elements - node.js

Question

I'm trying to populate a model with data from another model. Those two models looks like this:
var postSchema = mongoose.Schema({
_comments: { type: mongoose.Schema.Types.ObjectId, ref: 'Comment' },
type: String,
body: String,
});
var commentSchema = mongoose.Schema({
id_post: mongoose.Schema.Types.ObjectId,
body: String,
});
I want to find all posts and populate them with comments that have id_post == _id from founded Posts. Something like this:
Post.find({}).populate({
path: '_comments',
select: 'body',
match: { post_id: Post._id }
options: { limit: 5 }
})
.exec(function (err, posts){...});

Answer 1

First of all, There are few problems in the code you wrote.
If each post may have many comments you should implement one-to-many relationship between your schemas, you can do it by surrounding the comment ref with []
var postSchema = mongoose.Schema({
_comments: [ {type: mongoose.Schema.Types.ObjectId, ref: 'Comment'} ] ,
type: String,
body: String,
});
id_post is not just a field of type ObjectId, it should be written like this:
var commentSchema = mongoose.Schema({
post: { type: mongoose.Schema.Types.ObjectId, ref: 'Post' },
body: String,
});
When saving a new comment make sure you connect it to its post:
var comment = new Comment({
body: "Hello",
post: post._id // assign the _id from the post
});
comment.save(function (err) {
if (err) return handleError(err);
// thats it!
});
Now when you want to find a post and populate its comments you should write something like this:
Post
.find(...)
.populate({
path: '_comments',
select: 'body',
options: { limit: 5 }
})
.exec()
The reason I dropped the match is that match should be used when you want to filter according to a specific field, in your case you can use match to get only comments with type='something'.
populate should work because when you inserted the comment you made the bond to its post.
More information on the right way of using populate can be found here - Mongoose Query Population
Post data should be persisted the following way:
{
body: "some body",
type: "some type",
_comments: [12346789, 234567890, ...]
}
More information about the way the ref will be persisted here - One-to-Many Relationships with Document References