title: Rails migration depends on settings
body: <p>I have some migrations that depends on instance settings. If something is set some tables will join to one table, if not it will join to other table.</p>
  <p>Of course when I run the migration locally it will generate a db/schema.rb that can be different from the one expected on a server with different settings.</p>
  <p>Will that be an issue? Ie. for specs and deploys.</p>