title: Sequelize doesn't create a foreign key column on a populated table
body: <p>This is my popular example database :
  <a href="https://www.mysqltutorial.org/mysql-sample-database.aspx/" rel="nofollow noreferrer">https://www.mysqltutorial.org/mysql-sample-database.aspx/</a></p>
  <p>I need to create a customer_id foreign key in the payments table, using squelize :</p>
  <pre><code>  payments.belongsTo(customers, {
          foreignKey: 'customer_id',
      });
  </code></pre>
  <p>Unfortunatly, The foreignKey column is not automatically created by sequelize while it should, however, if I use this command :</p>
  <pre><code>sequelize.sync({ force: true })
  </code></pre>
  <p>The column is created, but my full database is content cleared !</p>
  <ol>
  <li>What is the solution to keep my actual data ?</li>
  <li>Do I have to create manually each foreign columns ?</li>
  <li>If yes, what is the exact syntax ?</li>
  </ol>
  <p>Thanks a lot !</p>
  <p>P.S : The tables are populated with examples datas. I  don't want to clear this data .
  P.S : I use <a href="https://github.com/sequelize/sequelize-auto" rel="nofollow noreferrer">https://github.com/sequelize/sequelize-auto</a> to generate data models from the existing database .</p>