title: "Join" a list of Observables in RxJava
body: <p>I have been reading the documentation over and over again and I cannot find the right operator for my case:</p>
  
  <pre><code> return Observable.fromIterable(formIds)
                              .flatMap(new Function&lt;String, Observable&lt;List&lt;Transmission&gt;&gt;&gt;() {
                                  @Override
                                  public Observable&lt;List&lt;Transmission&gt;&gt; apply(String formId) {
                                      return getFormTransmissions(formId);
                                  }
                              })
                              .toList()
                              .toObservable()
                              .map(new Function&lt;List&lt;List&lt;Transmission&gt;&gt;, List&lt;Transmission&gt;&gt;() {
                                  @Override
                                  public List&lt;Transmission&gt; apply(List&lt;List&lt;Transmission&gt;&gt; lists) {
                                      List&lt;Transmission&gt; transmissions = new ArrayList&lt;&gt;();
                                      for (List&lt;Transmission&gt; transmissionList : lists) {
                                          if (transmissionList != null) {
                                              transmissions.addAll(transmissionList);
                                          }
                                      }
                                      return transmissions;
                                  }
                              });
  </code></pre>
  
  <p>As you can see the last map() I take all the observables and combine their content. I don't like this part as I feel there must be some rxjava function to do that but I cannot find one.</p>
  
  <p>I will try to explain what the code does:
  From a list of ids
  For each one of them, get a list of transmissions for each id
  Once all of those complete combine all the individual transmission lists into one. Any ideas?</p>