title: Pandas merge adding column
body: <p>I have two dataframes <code>df1</code> and <code>df2</code>. <code>df1</code> contains the columns <code>subject_id</code> and <code>time</code> and <code>df2</code> contains the columns <code>subject_id</code> and <code>final_time</code>. What I want to do is for every <code>subject_id</code> in <code>df1</code> add a column with  <code>final_time</code> from <code>df2</code> but only from the <code>subject_ids</code>'s contained in <code>df1</code>. I have tried <code>df1.merge(df2,how='left')</code> but still get all of the <code>subject_id</code>'s from <code>df2</code> which is much longer and contains many duplicates of 'subject_id`.</p>
  
  <p>Example of what I am looking for:</p>
  
  <pre><code>df1
  
  
      subject_id  time  
  
   0          15  12:00 
   1          20  12:05 
   2          21  12:10
   3          25  12:00 
  
  df2
  
  
      subject_id  final_time 
  
   0          15       12:30      
   1          15       12:30      
   2          15       12:30      
   3          20       12:45      
   4          20       12:45      
   5          21       12:50      
   6          25        1:00       
   7          25        1:00       
   8          25        1:00      
  </code></pre>
  
  <p>What I am looking for</p>
  
  <pre><code>    subject_id  time  final_time 
  
   0          15  12:00      12:30      
   1          20  12:05      12:45      
   2          21  12:10      12:50      
   3          25  12:00       1:00       
  </code></pre>