Is there a way to return part of an array in a document in MongoDB?

Question

Pretend I have this document:

{
    "name": "Bob",
    "friends": [
        "Alice",
        "Joe",
        "Phil"
    ],
    "posts": [
        12,
        15,
        55,
        61,
        525,
        515
    ]
}

All is good with only a handful of posts. However, let's say posts grows substantially (and gets to the point of 10K+ posts). A friend mentioned that I might be able to keep the array in order (i.e. the first entry is the ID of the newest post so I don't have to sort) and append new posts to the beginning. This way, I could get the first, say, 10 elements of the array to get the 10 newest items.

Is there a way to only retrieve posts n at a time? I don't need 10K posts being returned, when most of them won't even be looked at, but I still need to keep around for records.

Vishwas · Accepted Answer · 2015-08-15 01:54:19Z

3

You can use $slice operator of mongoDB in projection to get n elements from array like following:

db.collection.find({
   //add condition here
  }, {
    "posts": {
    $slice: 3 //set number of element here
      //negative number slices from end of array
    }
})

answered Aug 15, 2015 at 1:54

Vishwas

7,0777 gold badges46 silver badges70 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Pratik Gujarathi · Accepted Answer · 2015-08-15 07:49:31Z

-1

You can do this : create a list for posts you want to have (say you want first 3 posts) and return that list

for doc in db.collections.find({your query}):
    temp = ()
    for i in range (2):
        temp.push(doc['posts'][i])
    return temp

answered Aug 15, 2015 at 7:49

Pratik Gujarathi

9651 gold badge11 silver badges21 bronze badges

1 Comment

serge1peshcoff Over a year ago

This will still do the big query to the DB

Collectives™ on Stack Overflow

Is there a way to return part of an array in a document in MongoDB?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related