text: 
  exports.up = function(knex, Promise) {
    return knex.schema.table('markers', function(table) {
      table.integer('city_id').unsigned().index();
      table.foreign('city_id')
        .references('id')
        .inTable('cities')
        .onDelete('RESTRICT')
        .onUpdate('CASCADE');
    });
  };
  
  exports.down = function(knex, Promise) {
    return knex.schema.table('markers', function(table) {
      table.dropColumn('city_id');
    });
  };
  