title: Rails Migration: Remove constraint
body: <p>I have a table in a Rails application which (in schema.rb) looks like:</p>
  
  <pre><code>create_table "users", :force =&gt; true do |t|
     t.string "name", :null=&gt;false
     t.string "address", :null=&gt;false
  end
  </code></pre>
  
  <p>I would like to write a rails migration to allow nulls for the address field. i.e. after the migration the table looks like this:</p>
  
  <pre><code>create_table "users", :force =&gt; true do |t|
     t.string "name", :null=&gt;false
     t.string "address"
  end
  </code></pre>
  
  <p>What do I need to do to remove the constraint?</p>