MongoDB (mongoose) retrieve substring

Question

Let's say I have a blog with some very long posts.

So, I want to display a list of my posts in "preview mode", for instance only first 50 chars of text.

Simple answer is to do this:

Post.find(
    (err, posts) => {
        if(err) return console.log(err);
        posts.forEach(
            post => {
                console.log('post preview:', post.data.substr(0,50) + '...');
            }
        )
    }
)

This way we retrieve all data from specific collection. If each post has more than 3 KB of data retrieving 30 posts seems very inefficient in terms of data transfer and processing.

So, I wondered if there is a way to retrieve already sliced string from DB?

Or at least do you have a better solution for my issue?

Answer 1

yes, you can use the $substr operator with a query like this :

db.collection.aggregate(
   [
     {
       $project:
          {
            preview: { $substr: [ "$data", 0, 50 ] }
          }
      }
   ]
)

Edit:

$substr is deprecated from mongodb 3.4 because it only has a well-defined behavior for strings of ASCII characters. If you're facing UTF-8 issues, consider upgrading to mongodb 3.4 in order to use the $substrCP operator so your query becomes :

db.collection.aggregate(
   [
     {
       $project:
          {
            preview: { $substrCP: [ "$data", 0, 50 ] }
          }
      }
   ]
)

As of today, mongodb 3.4 is only available for development, but a production version should be released soon