title: Counting M2M objects in Django
body: <p>I have the following model:</p>
  
  <blockquote>
    <p>class Task(Model):</p>
  
  <pre><code>solvers = ManyToManyField(User, related_name='slvrs')
  </code></pre>
  </blockquote>
  
  <p>Now I want to count the total number of solvers of all the tasks.
  For example there are 3 Task objects in the database. The first one has been solved by 2 users (M2M field relates to 2 users), the second by 3 and the last one by 0.
  I should get the count of all these solvers: 2 + 3 + 0 = 5.</p>
  
  <p>I believe I have to use Count aggregate, but I have know idea on which models and which annotations.</p>