title: Sequelize doesn't create foreign keys as constraints
body: <p>We're trying to create tables automatically inside PostgreSQL using Sequelize. Unfortunately, it doesn't create the foreign keys as constraints. Here is an example of one of my models:</p>
  
  <pre><code>module.exports = function(schema, DataTypes) {
  
      var definition = {
          id: {
              type: DataTypes.INTEGER,
              primaryKey: true,
              autoIncrement: true
          },
          deal_id: {
              type: DataTypes.INTEGER
          },
          image: DataTypes.STRING,
      };
  
  
      var image = schema.define('Image', definition, {
          tableName: 'images', // this will define the table's name
          timestamps: false, // this will deactivate the timestamp columns
          syncOnAssociation: true,
  
          classMethods: {
              getDefinition: function() {
                  return definition;
              },
              associate: function(_models) {
                  image.belongsTo(_models.Product, {
                      foreignKey: 'deal_id'
                  });
              }
          }
      });
  
      return image;
  };
  </code></pre>
  
  <p>Am I doing something wrong?</p>