title: replace column values in one dataframe by values of another dataframe
body: <p>I have two dataframes, the first one has 1000 rows and looks like:</p>
  <pre><code>Date            Group         Family       Bonus
  2011-06-09      tri23_1       Laavin       456
  2011-07-09      hsgç_T2       Grendy       679
  2011-09-10      bbbj-1Y_jn    Fantol       431
  2011-11-02      hsgç_T2       Gondow       569
  </code></pre>
  <p>The column <code>Group</code> has different values, sometimes repeated, but in general about 50 unique values.</p>
  <p>The second dataframe contains all these 50 unique values (50 rows) and also the hotels, that are associated to these values:</p>
  <pre><code>Group             Hotel
  tri23_1           Jamel
  hsgç_T2           Frank
  bbbj-1Y_jn        Luxy
  mlkl_781          Grand Hotel
  vchs_94           Vancouver
  </code></pre>
  <p>My goal is to replace the value in the column <code>Group</code> of the first dataframe by the corresponding values of the column <code>Hotel</code> of the second dataframe/or create the column <code>Hotel</code> with the corresponding values. When I try to make it just by assignment like</p>
  <pre><code>df1.loc[(df1.Group=df2.Group), 'Hotel']=df2.Hotel
  </code></pre>
  <p>I have an error that the dataframes are not of equal size, so the comparison is not possible.</p>