Elastic search order by count of nested object

Question

Beginner with Elasticsearch. I feel like this should be pretty simple, but I'm stuck here. I've got a mapping for Posts that looks like this:

[ post1: {
   title: 'asdfasd',
   comments: [commment1, comment2, comment3]
},
post2: {
   title: 'asdf',
   comments: [comment1, comment2]
}
.
.
.]

And I'm trying to search for them by title and then order them by number of comments. I can search by title just fine, but I'm a little confused as to how to go about ordering the results by comments count. What would be the best way to go about doing this?

Answer 1

You have two options -

1. Use a script to get the length of an array. So you would do something like:

   {  
     "query" : {  
       ....  
     },  
     "sort" : {  
       "_script" : {  
         "script" : "doc['comments'].values.length",  
         "type" : "number",  
         "order" : "desc"  
       }  
     }  
   }
   

2. Keep an additional field for the number of comments, each time you add a comment also increment the value of the comments counter, and sort by it.

Option #2 is preferable if you have a lot of data. Using a script has its overhead and it can increase search time if you have to calculate the script on a large collection of documents. Sorting by a field, on the other hand, is much better in terms of performance. I would go with #2.