text: exports.up = function(knex, Promise) {
      return Promise.all([
          knex.schema.table('customers', table => {
              table.string('uid')
                  .unique()
                  .notNullable();
          })
      ])
  };
  
  exports.down = function(knex, Promise) {
      return Promise.all([
          knex.schema.table('customers', table => {
              table.dropColumn('uid');
          })
      ])
  };