title: How to fill empty values in a dataframe based on columns in another dataframe?
body: <p>I have a dataframe called <code>df1</code>:</p>
  
  <pre><code>ID     Value       Name      Score
  -1      10           A         -1
  -1       5           B         -1
  NaN     0.2       Track C     100
  NaN     0.5       Track C     200
  1        0           D        100
  5        0           D        200
  </code></pre>
  
  <p>I want to fill the <code>NaN</code> in column <code>ID</code> with multiple rows of <code>Score</code> data from dataframe <code>df2</code>.</p>
  
  <p><code>df2</code>:</p>
  
  <pre><code>Score    ID
  100      1
  100      2
  100      3
  100      4
  200      5
  200      6
  200      7
  </code></pre>
  
  <p>So that ultimately, my final dataframe looks like this:
  <code>df3</code>:</p>
  
  <pre><code>ID     Value       Name      Score
  -1      10           A         -1
  -1       5           B         -1
  1       0.2       Track C     100
  2       0.2       Track C     100
  3       0.2       Track C     100
  4       0.2       Track C     100
  5       0.5       Track C     200
  6       0.5       Track C     200
  7       0.5       Track C     200
  1        0           D        100
  5        0           D        200
  </code></pre>
  
  <p>How could I accomplish this?</p>