title: Rails : migration for creating a fixed-length char(12) column
body: <p>What is the best way to define a fixed-length SQL column (CHAR(12) for instance) through a Rails migration ?</p>
  
  <p>Why this should not handled by the model is because of the performance of char() vs varchar(), and I'd like to avoid injecting raw SQL in the database.</p>
  
  <p>Edit : I know the :limit modifier, however the field is still varchar (which is bad for performance) and does not allow a minimum size.</p>