Mongodb findone document in array by object id

Question

Can I get only 1 photo by objectid? I only need to get 1 Image detail from 1 post by photo but what i get is all photo of post.

this is my db structure

and this is my code looks like:

 Post.findOne({
  $and: [
    { photo: { $elemMatch: { _id: id } } }       ]
}).exec((err, post) => {
  if (err) {
    return res.status(400).json({ error: err });
  }
  req.post = post;
  console.log(req.post);
  next();
});

what i get in req.post is only [].

Thanks in advance

If you want to get a photo by an id, regardless of what post it is, you can do { 'post.photo._id': id } in your search condition — Tizio Fittizio
– Tizio Fittizio, Commented Sep 15, 2019 at 9:00
I still get all photos. Is it posible to get only 1 from photos by photo objectId ? — kaks
– kaks, Commented Sep 15, 2019 at 12:00
Im finally getting this one [{ "photo": [{ "_id": "5d7dd20185251921882e2ba1", "data": "", "contentType":"" }] }] Now my problem is I cant get the contentType. undefined on post.photo — kaks
– kaks, Commented Sep 18, 2019 at 14:07

ambianBeing · Accepted Answer · 2020-02-13 10:18:55Z

3

The $elemMatch projection operator provides a way to alter the returned documents, in here coupled with find utilizing second parameter which is projection will achieve that.

Post.find(
  {},
  {
    _id: 0,
    photo: { $elemMatch: { _id: id } }
  }
);

This will provide leaned documents with the promise: .lean().exec():

Post.find(
  {},
  {
    _id: 0,
    photo: { $elemMatch: { _id: id } }
  }
)
  .lean()
  .exec();

NOTE: $elemMatch behaviour is to return the first document where photo's id matches.

edited Feb 13, 2020 at 10:18

answered Sep 15, 2019 at 19:15

ambianBeing

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2 Comments

kaks Over a year ago

It gives me this [ { photo: [ [Object] ] } ]. I cant get contentType

ambianBeing Over a year ago

@kaks That object should contain all properties. You could try with .lean().exec().

Dmitriy Botov · Accepted Answer · 2019-09-15 18:07:11Z

1

You can try with aggregate instead of findOne: https://docs.mongodb.com/manual/reference/operator/aggregation-pipeline/

Post.aggregate([
    { $match: { 'photo._id': id } },
    { $unwind: "$photo" },
    { $match: { 'photo._id': id } },
]);

Maybe not the best, but single photo data is achievable.

answered Sep 15, 2019 at 18:07

Dmitriy Botov

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