title: Merging two dataframes of different length, on a particular column with different number of instances
body: <p>I have two dataframes D1 and D2. Both are of different sizes but have some relation. D1 has a column "ID" has has multiple instances of the same ID and D2 has only a single instance. So how can I concatenate these two dataframes together?</p>
  
  <pre><code>    D1:                             D2:
      ID  val1  val2                  ID   Target   
      1    x     y                    1      0
      1    x     y                    2      1
      2    a     b
      2    a     c
  </code></pre>
  
  <p>So here I need to have a new column in D1 called as target for every row in D1 with the corresponding value from D2</p>
  
  <pre><code>    ID  val1  val2 Target   
      1    x     y     0
      1    x     y     0
      2    a     b     1
      2    a     c     1
  </code></pre>