Nested field_value_factor on individual field

Elastic Stack Elasticsearch
1

I have some document with mapping looks like following:

    {  
   "documents":[  
      {  
         "author":"name1",
         "books":[  
            {  
               "title":"title1",
               "content":"content1",
               "rating":5
            },
            {  
               "title":"title2",
               "content":"content2",
               "rating":1
            }
         ]
      },
      {  
         "author":"name2",
         "books":[  
            {  
               "title":"title1",
               "content":"content1",
               "rating":3
            },
            {  
               "title":"title2",
               "content":"content2",
               "rating":3
            }
         ]
      }
   ]
}

I want to return a list of authors whose book contents match certain keywords. But at the same time, I want the score on each documents.books.content is boosted by documents.books.rating, then combine these score together.