title: r2dbc validate if id exists and then insert row
body: <p>I have following sql query with r2dbc:</p>
  <pre><code>      client.sql(&quot;INSERT INTO relation(id,rel_id) &quot;
              + &quot;VALUES(:id,:rel_id)&quot;)
        .filter((statement, executeFunction) -&gt; statement.returnGeneratedValues(&quot;id&quot;).execute())
        .bind(&quot;id&quot;, id)
        .bind(&quot;rel_id&quot;, rel_id)
        .fetch()
        .first()
        .subscribe(
                data -&gt; log.info(&quot;inserted data : {}&quot;, data),
                error -&gt; log.error(&quot;error: {}&quot;, error)
        );
  </code></pre>
  <p>And my problem is thet rel_id is constraint(foreign key) to another table Person. So, I would like to check if rel_id exists in table Person and if exists then insert into relation. Or maybe throw some exception if not founded in Person table. I don't know what is prefered way for this case.</p>