title: Elasticsearch "More Like This" API vs. more_like_this query
body: <p>Elasticsearch has two similar features to get "similar" documents:</p>
  
  <p>There is the <a href="http://www.elasticsearch.org/guide/reference/api/more-like-this.html" rel="noreferrer">"More Like This API"</a>. It gives me documents similar to a given one. I can't use it in more complex expressions though.</p>
  
  <p>There is also the <a href="http://www.elasticsearch.org/guide/reference/query-dsl/mlt-query.html" rel="noreferrer"><code>"more_like_this"</code> query for use in the Search API</a> I can use it in bool or boosting expressions, but I can't give it an id of a document. I have to provide the <code>"like_text"</code> parameter.</p>
  
  <p>I have documents with tags and content. Some documents will have good tags and some won't have any. I want a "Similar documents" feature that will work every time but will rank documents with matching tags higher than documents with matching text. My idea was:</p>
  
  <pre><code>{
      "boosting" : {
          "positive" : {
              "more_like_this" : {
                  "fields" : ["tag"],
                  "id" : "23452",
                  "min_term_freq" : 1
              }
          },
          "negative" : {
              "more_like_this" : {
                  "fields" : ["tag"],
                  "id" : "23452",
              }
          },
          "negative_boost" : 0.2
      }
  }
  </code></pre>
  
  <p>Obviously this doesn't work because there is no <code>"id"</code> in <code>"more_like_this"</code>. What are the alternatives?</p>