How to sort search results according to the number of elements in ElasticSearch?

Question

Let's say that I am storing documents like this in ElasticSearch:

{ 'name':'user name', 'age':43, 'location':'CA, USA', 'bio':'into java, scala, python ..etc.', 'tags':['java','scala','python','django','lift'] } 

And let's say what I'm looking for using location = CA, how can I sort the results according to the number of elements in the 'tags'?

I would like to list the people with the most tags on the first page.

Answer 1

You can do this by indexing an additional field that contains the number of tags on which you can easily sort the results. Otherwise, if you are willing to pay a small execution cost during the request, there is a good solution that does not require reindexing your data: you can sort based on the script as follows:

 { "query" : { "match_all" : {} }, "sort" : { "_script" : { "script" : "doc['tags'].values.length", "type" : "number", "order" : "asc" } } } 

As you can read the script based sorting section:

Note. It is recommended for a separate script-based sort based on user preferences, use the custom_score query instead, since evaluation sorting is faster.

This means that it would be better to use a custom counting query to affect your score and then sort by the result, for example:

 { "query" : { "custom_score" : { "query" : { "match_all" : {} }, "script" : "_score * doc['tags'].values.length" } } }