code: function (columns, opts) {
              opts = opts || {};
              this.constraints.push(merge({type:"unique", columns:toArray(columns)}, opts));
          }
docs: Add a unique constraint on the given columns to the DDL. See {@link patio.SchemaGenerator#constraint}
  for argument types.
  @example
  DB.createTable("test", function(){
  this.unique("name");
  //=> UNIQUE (name)
  });