title: Two entities should have same parent entity in relational model
body: <p>I have simple models: People, Photos, Pair of photos.</p>
  <p>An instance of &quot;Pair of photos&quot; should be linked with two &quot;Photo&quot; instances which are linked with the same &quot;Human&quot; instance.</p>
  <p>I tried to implement it using ER model. In <a href="https://i.stack.imgur.com/Vnhqb.png" rel="nofollow noreferrer">this</a> case, &quot;PhotoPair&quot; can contain photos from different persons, so it is wrong.</p>
  <p>So, how I can solve this problem? (without using triggers)</p>