ActiveRecord::StatementInvalid in UsersController#create (Insert does not include newly added column)

I'm presently teaching myself ruby on rails following my degree. I'm really new to the language and want to know more about how migrations work.

[Form the User uses][1]

[Error of Problem][2]

I have updated my existing users table through migration to allow for an additional column 'username' and included some HTML/Ruby to allow the user to enter in the field. I can't fathom how rails generates INSERT SQL statements or updates them upon migration.

Below is my Code for HTML

<%= form_for(user) do |f| %>
  <% if user.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(user.errors.count, "error") %> prohibited this user from being saved:</h2>

      <ul>
      <% user.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :username %>
    <%= f.text_field :username %>
  </div>


  <div class="field">
    <%= f.label :password %>
    <%= f.text_field :password %>
  </div>

  <div class="field">
    <%= f.label :email %>
    <%= f.text_field :email %>
  </div>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

This is the schema after migration

[Schema after migration (Users)][1]

I must be missing something...