title: Update some specific field of an entity in android Room
body: <p>I am using android room persistence library for my new project.
  I want to update some field of table. 
  I have tried like in my <code>Dao</code> -</p>
  
  <pre><code>// Method 1:
  
  @Dao
  public interface TourDao {
      @Update
      int updateTour(Tour tour);
  }
  </code></pre>
  
  <p>But when I try to update using this method then it updates every field of the entity where it matches primary key value of tour object.
  I have used <code>@Query</code></p>
  
  <pre><code>// Method 2:
  
  @Query("UPDATE Tour SET endAddress = :end_address WHERE id = :tid")
  int updateTour(long tid, String end_address);
  </code></pre>
  
  <p>It is working but there will be many queries in my case because I have many fields in my entity. I want to know how can I update some field (not all) like <code>Method 1</code> where id = 1; (id is the auto generate primary key).</p>
  
  <pre><code>// Entity:
  
  @Entity
  public class Tour {
      @PrimaryKey(autoGenerate = true)
      public long id;
      private String startAddress;
      private String endAddress;
      //constructor, getter and setter
  }
  </code></pre>