title: Filter Dask Dataframe on categorical column?
body: <p>Say I have a large dask dataframe of fruit.  I have thousands of rows but only about 30 unique fruit names, so I make that column a category:</p>
  
  <pre><code>df['fruit_name'] = df.fruit_name.astype('category')
  </code></pre>
  
  <p>Now that this is a category, can I no longer filter it?  For instance,</p>
  
  <pre><code>df_kiwi = df[df['fruit_name'] == 'kiwi']
  </code></pre>
  
  <p>will return TypeError("invalid type comparison")</p>
  
  <p>If I try to create a "dummy" dataframe and merge against that, I get a ValueError: "You are trying to merge on int8 and category columns..."</p>
  
  <pre><code>df_dummy = pd.DataFrame(data={'fruit_name': 'kiwi'}, index=range(1))
  df_dummy['fruit_name'] = df_dummy.fruit_name.astype('category')
  
  df_new = df.merge(df_dummy, how="inner", on="fruit_name")
  </code></pre>
  
  <p>Do I lose certain merge and filter functionality on a categorical column, or am I just doing this wrong (I am still extremely new to dask and pandas).  Thanks!</p>