title: sqlalchemy constraint on a boolean column
body: <p>I have a table with 3 columns A(, B, C) and have 2 constraints</p>
  
  <ol>
  <li>A and B should be jointly unique which I have done by defining a unique  constraint. </li>
  <li>For any given value in A, there can be only one True in C. (A, C=True) has to be unique. 
  I need help in defining the second constraint. </li>
  </ol>
  
  <p>e.g Data set</p>
  
  <h2>A      :         B              C</h2>
  
  <p>a1      :        b1      :       False</p>
  
  <p>a1       :       b2     :        True</p>
  
  <p>a1        :      b3       :      False</p>
  
  <p>a2         :     b1      :       True</p>
  
  <p>a2          :    b2      :       False</p>
  
  <p>a2           :   b3   :          False</p>