title: Spring / Reactor : Retrieve and modify a Mono
body: <p>I have the following model :</p>
  
  <pre><code>class Person {
      List&lt;Address&gt; addresses;
  }
  </code></pre>
  
  <p>I used the <code>ReactiveMongoRepository</code> to retrieve a Mono.</p>
  
  <pre><code>public interface PersonRepository extends ReactiveMongoRepository&lt;Person, String&gt;{
  }
  </code></pre>
  
  <p>But I can't find a way to add an Address to the person's addresses
   and return a Mono of the given Address.
  Here is the signature of what I want to achieve :</p>
  
  <pre><code>Mono&lt;Address&gt; addAddress(Address address)
  </code></pre>
  
  <p>Here is the code I used to use without Reactor :</p>
  
  <pre><code>public Mono&lt;Addresses&gt; addAddress(Address address){
      Mono&lt;Person&gt; person = personRepository.findById(personId);
      // person.getAddresses().add(address) ?
      // personRepository.save(person) ?
      // return "address" as Mono
  }
  </code></pre>
  
  <p><strong>Question:</strong> How can I add an Address to a <code>Mono&lt;Person&gt;</code> and then return a Mono of that Address ?</p>