title: doctrine schema update always asking for constraints and indexes
body: <p>i work with symfony on ubuntu 12.04,
  when i operate a doctrine schema update from command line,
  the dump always prints out that he has to make things like this:</p>
  
  <pre><code>ALTER TABLE melu_numeros ADD CONSTRAINT FK_8062E62AFB539063 FOREIGN KEY (id_fan) REFERENCES melu_fanzine (id_fan);
  ALTER TABLE Image ADD CONSTRAINT FK_4FC2B5BABEDD0E7 FOREIGN KEY (id_num_fan) REFERENCES melu_numeros (id_num_fan);
  CREATE INDEX IDX_4FC2B5BABEDD0E7 ON Image (id_num_fan);
  DROP INDEX IDX_7B0247F15D172A78 ON melu_categories;
  ALTER TABLE melu_categories ADD id_num_fan INT NOT NULL, DROP numero_id;
  ALTER TABLE melu_categories ADD CONSTRAINT FK_7B0247F1ABEDD0E7 FOREIGN KEY (id_num_fan) REFERENCES melu_numeros (id_num_fan);
  CREATE INDEX IDX_7B0247F1ABEDD0E7 ON melu_categories (id_num_fan);
  ALTER TABLE melu_fichiers ADD CONSTRAINT FK_E2A048FCD6E65D60 FOREIGN KEY (id_agen) REFERENCES melu_agenda (id_agen);
  ALTER TABLE melu_fanzine ADD CONSTRAINT FK_99AC1D58A656D38D FOREIGN KEY (id_asso) REFERENCES melu_association (id_asso);
  ALTER TABLE melu_tarifs ADD CONSTRAINT FK_409B4C8D6E65D60 FOREIGN KEY (id_agen) REFERENCES melu_agenda (id_agen);
  ALTER TABLE melu_newsletter CHANGE email email VARCHAR(50) NOT NULL;
  ALTER TABLE melu_commentaire ADD CONSTRAINT FK_EE62B25DABEDD0E7 FOREIGN KEY (id_num_fan) REFERENCES melu_numeros (id_num_fan);
  ALTER TABLE melu_contenus DROP INDEX FK_D9DCDC6130453DE8, ADD UNIQUE INDEX UNIQ_D9DCDC6130453DE8 (type_cont);
  ALTER TABLE melu_contenus ADD CONSTRAINT FK_D9DCDC6130453DE8 FOREIGN KEY (type_cont) REFERENCES melu_types (id_types);
  CREATE UNIQUE INDEX id_cont ON melu_contenus (id_cont);
  ALTER TABLE melu_types ADD CONSTRAINT FK_8AB9AD2AB7B5C800 FOREIGN KEY (id_types) REFERENCES melu_contenus (type_cont);
  ALTER TABLE melu_photos ADD CONSTRAINT FK_F5C79087D6E65D60 FOREIGN KEY (id_agen) REFERENCES melu_agenda (id_agen);
  ALTER TABLE melu_jour ADD CONSTRAINT FK_85C738A2D6E65D60 FOREIGN KEY (id_agen) REFERENCES melu_agenda (id_agen);
  DROP INDEX IDX_6C66C2856D2F0F48 ON melu_util_droit;
  ALTER TABLE melu_util_droit DROP useur_id;
  ALTER TABLE melu_util_droit ADD CONSTRAINT FK_6C66C28550EAE44 FOREIGN KEY (id_utilisateur) REFERENCES melu_utilisateurs (id_util);
  </code></pre>
  
  <p>and when i force the update, i have this error:</p>
  
  <pre><code>[Doctrine\DBAL\DBALException]                                                                                                   
  An exception occurred while executing 'ALTER TABLE Image ADD CONSTRAINT FK_4FC2B5BABEDD0E7 FOREIGN KEY (id_num_fan) REFERENCES  melu_numeros (id_num_fan)':
  SQLSTATE[HY000]: General error: 1005 Can't create table 'meluzineorg1.#sql-5cf_438' (errno: 150)
  </code></pre>
  
  <p>the 2 entites Image and melu_numero that provke the error
  <a href="http://pastie.org/9100202" rel="nofollow">http://pastie.org/9100202</a></p>
  
  <p>how could i make this all work well ?</p>