Mongoose nested populate with select and limit - mongodb

Question

How can I limit the number of pictures?
CommentBy is a ref in Comment, which is a ref in Post.
const posts = await Post.find(filter).populate({
path: 'comments',
populate: {
path: 'commentBy',
select: 'first_name last_name pictures',
},
});
Post:
const postsSchema = new Schema(
{
text: String,
comments: { type: [ObjectId], ref: 'Comment' }, // comment ref here
reactions: {
type: [ObjectId],
ref: 'Reaction',
},
},
);
Comment:
const commentsSchema = new Schema({
commentBy: {
type: ObjectId,
ref: 'User',
},
text: {
type: String,
},
});
User:
const userSchema = new Schema({
first_name: {
type: String,
},
last_name: {
type: String,
},
pictures: {
type: [String],
},
});

Answer 1

I don't think that in mongoose you can limit a specific field of a populated document.
You could try to use lean to return an array of objects and manipulate that in plain JS:
let posts = await Post.find(filter).populate({
path: 'comments',
populate: {
path: 'commentBy',
select: 'first_name last_name pictures',
},
}).lean().exec();
posts = posts.map((post) => {
const pics = post.comments.commentBy.pictures
if (pics.length <= 10) return post;
post.comments.commentBy.pictures = pics.slice(0, 10);
return post;
})