How to effectively filter a nested array in a Mongoose pre-find query

Question

How to effectively filter a nested array in a Mongoose pre-find query

I am facing a particular issue. I have comments in my database that can be soft-deleted using a flag is_deleted. When this flag is set to true, it means the comment has been deleted.

The comments are not stored as an independent model; instead, they are nested within another model (simplified model):

let CommentSchema = new Schema({
    text: {
        type: String,
        required: true
    },
    modified_at: {
        type: Date,
        default: null
    },
    created_at: {
        type: Date,
        default: Date.now
    },
    is_deleted: {
        type: Boolean,
        default: false
    },
});

let BookSchema = new Schema({
    ...
    comments: [CommentSchema],
    ...
});

When retrieving all Books with

Books.find({}, (err, books) => {})

, I want to exclude the deleted comments:

BookSchema.pre('find', function() {
    this.aggregate(
    {},
    { $unwind: '$comments'},
    { $match: {'comments.is_deleted': false}})
});

However, this does not seem to work. Do you have any suggestions on how to modify the query so that only non-deleted nested comments are returned without having a separate Comment collection?

EDIT: Just to clarify, I am accessing a specific book object through an endpoint like this: /books/{bookId}/comments. It would also be beneficial if this filtering could be applied when fetching all book objects.

node.js mongodb express mongoose mongoose-schema

Answer 1

Answer №1

If you're looking to filter sub-document elements within a nested array in MongoDB, the positional $ operator is your best bet. Check out this detailed explanation by @Blakes Seven Here

BookSchema.find({
 'comments.is_deleted': false,
}, {
 'comments.$': 1,
});

UPDATE:

While the $ operator only returns one element at a time, refer to this excerpt from the documentation :

The positional $ operator limits the query results to include only the first matching element

You have two options to tackle this issue:

Use an aggregate operation, although it may be slower due to database processing
Retrieve all books and filter their comments using loops

BookSchema.find({
   'comments.is_deleted': false,
}).map(x => Object.assign(x, {
   comments: x.comments.filter(y => !y.is_deleted),
}));

The find method fetches books with non-deleted comments.

The map function iterates over each book.

We then remove any deleted comments before returning the result.

Answer 2

If you're looking to filter sub-document elements within a nested array in MongoDB, the positional $ operator is your best bet. Check out this detailed explanation by @Blakes Seven Here

BookSchema.find({
 'comments.is_deleted': false,
}, {
 'comments.$': 1,
});

UPDATE:

While the $ operator only returns one element at a time, refer to this excerpt from the documentation :

The positional $ operator limits the query results to include only the first matching element

You have two options to tackle this issue:

Use an aggregate operation, although it may be slower due to database processing
Retrieve all books and filter their comments using loops

BookSchema.find({
   'comments.is_deleted': false,
}).map(x => Object.assign(x, {
   comments: x.comments.filter(y => !y.is_deleted),
}));

The find method fetches books with non-deleted comments.

The map function iterates over each book.

We then remove any deleted comments before returning the result.

How to effectively filter a nested array in a Mongoose pre-find query

Answer №1

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