title: Declaring a retrofit REST endpoint with constant query value
body: <p>So I want to get the metadata of a youtube video (say this one: <code>https://www.youtube.com/watch?v=qlTA3rnpgzU</code>). </p>
  
  <p>I'm going to encode it and wrap it in another url like so: <code>http://www.youtube.com/oembed?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DqlTA3rnpgzU&amp;format=json</code></p>
  
  <p>My interface definition will look like this:</p>
  
  <pre><code>public interface YoutubeApi {
      @GET ("oembed") 
      YoutubeMetaData metaData (@Query (QUERY_VIDEO_URL) final String url,
                                @Query(QUERY_FORMAT) final String alwaysJson);
  }
  </code></pre>
  
  <p>That's all fine and dandy, but I don't ever want to specify any format other than JSON here (<em>format=json is a fixed part of this url</em>).</p>
  
  <p>Is there a way to specify this in my interface declaration and reduce my interface to:</p>
  
  <pre><code>public interface YoutubeApi {
      @GET ("oembed") 
      @Magic ("format=json")
      YoutubeMetaData metaData (@Query (QUERY_VIDEO_URL) final String url);
  }
  </code></pre>
  
  <p>Thanks.</p>