title: r2dbc ReactiveCrudRepository how to write JPQL/HQL
body: <p>I'm new to Spring WebFlux reactive. I use R2DBC postgresql. I have a repository like that:</p>
  <pre><code>public interface BookRepository extends ReactiveCrudRepository&lt;Book, Long&gt; {
  }
  </code></pre>
  <p>Now I want to add custom method to query by many complicated conditions:</p>
  <pre><code>public interface CustomBookRepository extends BookRepository {
      Flux&lt;Book&gt; findByVeryComplicatedCondition(MyCriteriaDto criteria);
  }
  </code></pre>
  <p>My implementation:</p>
  <pre><code>public class CustomBookRepositoryImpl extends CustomBookRepository {
  
      //How to get it?
      EntityManager em;
  
      @Override
      public Flux&lt;Book&gt; findByVeryComplicatedCondition(MyCriteriaDto criteria) {
          Query query = em.createQuery(&quot;SELECT b from Book b WHERE (VERY COMPLICATED CONDITIONS)&quot;);
          //What next?
      }
  }
  </code></pre>
  <p>My questions are in the code above:</p>
  <ol>
  <li>How to obtain an EntityManager?</li>
  <li>How to get Flux from HQL query I built?</li>
  </ol>
  <p>When I ask these questions, I mean &quot;How to do this with Spring reactive/r2dbc way&quot;, not &quot;How to do this normal way with JDBC&quot;</p>