title: Populate/adjust new column of DataFrame from another DataFrame column by non-unique first DF column
body: <p>I have some DataFrame <code>df1</code> without index like this:</p>
  <pre><code>[
      &quot;event_id&quot;, &quot;cat_id&quot;, &quot;clm_1&quot;, ..., &quot;clm_n&quot;
       123,        345,     &quot;qwe&quot;,   ..., &quot;asd&quot;
       999,        222,     &quot;sss&quot;,   ..., &quot;ddd&quot;
       ...
       333,        345,     &quot;qqq&quot;,   ..., &quot;ooo&quot;
  ]
  </code></pre>
  <p>And another DataFrame <code>df2</code> (<code>cat_id</code> is index):</p>
  <pre><code>[
      &quot;cat_id&quot;, &quot;features&quot;
      345, &quot;cat,dog&quot;
      567, &quot;chupacabra&quot;
      ...
  ]
  </code></pre>
  <p><code>cat_id</code> in <code>df1</code> is non-unique, but it is unique in <code>df2</code> and it is an <code>index</code></p>
  <p>How can I add new <code>features</code> column to <code>df1</code> and populate it from <code>df2</code> by <code>cat_id</code>?</p>
  <p>Expected result is:</p>
  <pre><code>[
      &quot;event_id&quot;, &quot;cat_id&quot;, &quot;clm_1&quot;, ..., &quot;clm_n&quot;, &quot;features&quot;
       123,        345,     &quot;qwe&quot;,   ..., &quot;asd&quot;,   &quot;cat,dog&quot;
       999,        222,     &quot;sss&quot;,   ..., &quot;ddd&quot;,   None
       ...
       333,        345,     &quot;qqq&quot;,   ..., &quot;ooo&quot;,   &quot;cat,dog&quot;
  ]
  </code></pre>