title: Add column to pandas dataframe indexed by another column
body: <p>I have a pandas dataframe that looks like</p>
  
  <pre><code>        some_data  another_id
  id                                                            
  312871     ...     abc
  312977     ...     def
  ...
  </code></pre>
  
  <p>and a dictionary <code>letters = {"abc": "x", "def": "y"}</code>. I want to add <code>letters</code> as a column to my dataframe, but it is morally indexed by <code>another_id</code> so I can't just set <code>df["letters"] = letters</code>.</p>
  
  <p>I'm sure there's a beautiful, elegant way to do this. What is it?</p>