title: Get all discussions where a discussion has at least two active participants?
body: <p>I have a simple one-on-one chat program, and I'd like to find out the discussions where both participants have sent at least one message. How would I form such a query?</p>
  
  <p>The tables are like this (simplified):</p>
  
  <pre><code>Table USER
  - id
  - username
  
  Table DISCUSSION
  - id
  
  Table MESSAGE
  - id
  - user_id (the user who sent the message)
  - discussion_id (the discussion this message belongs to)
  </code></pre>
  
  <p>So essentially, I need to check that there's at least two <code>MESSAGE</code> rows that have the same <code>discussion_id</code> but <code>different user_id</code>'s.</p>