Using "deleteMany" post middleware in mongoose - node.js

Question

In my Nodejs and Express app, I have a mongoose User schema, a Post schema and a Comment schema as follows:
const UserSchema = new Schema({
username: {
type: String,
required: true,
unique: true
},
password: String,
posts : [
{
type : mongoose.Schema.Types.ObjectId,
ref : 'Post'
}
]
});
const PostSchema = new Schema({
author : {
type : mongoose.Schema.Types.ObjectId,
ref : 'User'
},
createdAt: { type: Date, default: Date.now },
text: String,
comments : [
{
type : mongoose.Schema.Types.ObjectId,
ref : 'Comment'
}
],
});
const CommentSchema = new Schema({
author : {
type : mongoose.Schema.Types.ObjectId,
ref : 'User'
},
createdAt: { type: Date, default: Date.now },
text: String
});
I have coded the general CRUD operations for my User. When deleting my user, I can easily delete all posts associated with that user using deleteMany:
Post.deleteMany ({ _id: {$in : user.posts}});
To delete all the comments for all the deleted posts, I can probably loop through posts and delete all the comments, but I looked at mongoose documentation here and it seems that deleteMany function triggers the deleteMany middleware. So In my Post schema, I went ahead and added the following after defining schema and before exporting the model.
PostSchema.post('deleteMany', async (doc) => {
if (doc) {
await Comment.deleteMany({
_id: {
$in: doc.comments
}
})
}
})
When deleting user, this middleware is triggered, but the comments don't get deleted. I got the value of doc using console.log(doc) and I don't think it includes what I need for what I intend to do. Can someone tell me how to use the deleteMany middleware properly or if this is not the correct path, what is the most efficient way for me to delete all the associated comments when I delete the user and their posts?

Answer 1

deleteMany will not give you access to the affected document because it's a query middleware rather than a document middleware (see https://mongoosejs.com/docs/middleware.html#types-of-middleware). Instead it returns the "Receipt-like" object where it tells it successfully deleted n objects and such.
In order for your hook to work as expected, you'll need to use something other than deleteMany, such as getting all of the documents (or their IDs), and loop through each one, using deleteOne.