title: HibernateJpaVendorAdapter's generateDdl doesn't alter tables
body: <p>I am developing a website using Spring+JPA+Hibernate. In the persistence configuration (JPA+Hibernate) I'm setting the HibernateJpaVendorAdapter's generateDdl attribute to true and in fact new entities correctly create the new table in the DB.</p>
  
  <p>Once the table has been created, though, if I add an attribute to the entity, I'm expecting the HibernateJpaVendorAdapter to alter the table and add the column as well. This is not happening and it's strange because in the Java AbstractJpaVendorAdapter's setGenerateDdl method documentation there is: "Set whether to generate DDL after the EntityManagerFactory has been initialized, creating/<strong>updating</strong> all relevant tables."</p>
  
  <p>JPA+Hibernate configuration:</p>
  
  <pre><code>&lt;bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"&gt;
      &lt;property name="driverClassName" value="${db.driverClassName}" /&gt;
      &lt;property name="url" value="${db.url}" /&gt;
      &lt;property name="username" value="${db.username}" /&gt;
      &lt;property name="password" value="${db.password}" /&gt;
  &lt;/bean&gt;
  
  &lt;bean id="jpaAdapter" class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter"&gt;
      &lt;property name="databasePlatform" value="org.hibernate.dialect.MySQL5Dialect" /&gt;
      &lt;property name="showSql" value="${db.showSql}" /&gt;
      &lt;property name="generateDdl" value="${db.generateDdl}" /&gt;
  &lt;/bean&gt;
  
  &lt;bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"&gt;
      &lt;property name="jpaVendorAdapter" ref="jpaAdapter" /&gt;
      &lt;property name="jpaProperties"&gt;
          &lt;props&gt;
              &lt;prop key="hibernate.show_sql"&gt;true&lt;/prop&gt;
              &lt;prop key="hibernate.format_sql"&gt;true&lt;/prop&gt;
          &lt;/props&gt;
      &lt;/property&gt;
      &lt;property name="dataSource" ref="dataSource" /&gt;
  &lt;/bean&gt;
  </code></pre>
  
  <p>persistence.xml:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;persistence
          xmlns="http://java.sun.com/xml/ns/persistence"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
     http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
          version="1.0"&gt;
  
      &lt;persistence-unit name="m8tsPU" /&gt;
  &lt;/persistence&gt;
  </code></pre>
  
  <p>Am I doing something wrong?</p>