Way to get summary of content

[jaru_kim (kyunghwan kim)]

I am testing query for index that has documents.

    {
      "query": {
        "bool": {
          "should": [
            {
              "match": {
                "title": "test.docx"
              }
            },
            {
              "match": {
                "content": "test.docx"
              }
            }
          ]
        }
      },
      "_source": ["title", "content"], 
     "highlight": {
       "fields": {
         "content": {}
        }
      }
    }

Query is to search keyword in title and content.
But, when keyword is matched only title of document, highlight is empty.
In this case, I want to get summary of content in search result.
for example of summary, front of 100 characters in content field.

I think that simple way is add mapping 'summary' field.
But Is there other way in query? not to add field.