Get only the last element of array mongoose

Question

I have array in a document, and I try to receive the last element of this array.

My code is:

Post.find({_id:postId},{'comments':{'$slice':-1}});

this gives me all the object but the comments array contains only the last element.

on the other hand,

Post.find({_id:postId},{'comments':1});

give me only the comments.

I dont find how to combine the two commands together. How it can be done?

{
 "users":[],
 "comments":["string1","string2","string3"],
 "lastValue":"Wow"
 "name":"jow"
 "_id": {
    "$oid": "5747d6bdecfae9d0560077cc"
   },
}

Thanks

What's your objective? From which array you are trying to retrieve last element? — Sandesh
– Sandesh, Commented May 27, 2016 at 5:52

oleh.meleshko · Accepted Answer · 2016-05-27 06:38:39Z

6

You might want to use mongodb (version 3.2) aggregation $slice like that:

Post.aggregate([
  { 
    $match: { 
      '_id.$oid': postId 
    }
  },
  { 
    $project: {
      comments: {
        $slice: [ "$comments", -1 ] 
      }
    }
  }
]);

In earlier versions of mongodb:

Post.aggregate([
  { 
    $match: { 
      '_id.$oid': postId 
    }
  },
  { 
    $unwind: "$comments"
  },
  {
    $group : {
      _id: "$_id.$oid",
      comment: { $last: "$comments" }
    }
  }
]);

edited May 27, 2016 at 6:38

answered May 27, 2016 at 6:11

oleh.meleshko

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4 Comments

Alon Over a year ago

This is almost works, I use the second, but their is problem with the _id it dont find the _id. if I remove the _id:postId, I receive the last of every element..

oleh.meleshko Over a year ago

@Alon in your example you search { _id: postId }, but in the given example there is $oid. All you need is to specify valid collection identifier in the $group _id field and $match criteria. Updated the answer.

Alon Over a year ago

Your fix dont work, I found that use , _id:mongoose.Types.ObjectId(postId) works

oleh.meleshko Over a year ago

@Alon oh, I couldn't know what you have in postId, makes sense too.

Zsolt Meszaros · Accepted Answer · 2022-01-06 15:59:53Z

5

I hope this helps.

db.Post.find(
  { _id: postId },
  { comments: { $slice: -1 }, _id: 0, users: 0, lastValue: 0, name: 0 },
);

edited Jan 6, 2022 at 15:59

Zsolt Meszaros

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answered May 27, 2016 at 6:05

Sandesh

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1 Comment

Alon Over a year ago

I hoped that their is something simplest, cause if I add things to the scheme I have to change this line also.

gprathour · Accepted Answer · 2020-04-24 01:10:30Z

4

In case of Mongoose, slice can work this way also,

model.find({ 
    // condition
})
.select('fields')
.slice('array', -1) // <------ Here
.then((data) => {
    // handle
})
.catch();

Just wrote pseudo code, as it might help someone.

answered Apr 24, 2020 at 1:10

gprathour

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Comments

Muhammad Talha · Accepted Answer · 2022-01-05 19:44:31Z

0

db.collection_name.find({'name':'how'},{'comments': {$slice: -1}})

answered Jan 5, 2022 at 19:44

Muhammad Talha

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