docs: def _model_columns(ins):
      """ Get columns info
  
      :type ins: sqlalchemy.orm.mapper.Mapper
      :rtype: list[SaColumnDoc]
      """
      columns = []
      for c in ins.column_attrs:
          # Skip protected
          if c.key.startswith('_'):
              continue
  
          # Collect
          columns.append(SaColumnDoc(
              key=c.key,
              doc=c.doc or '',
              type=str(c.columns[0].type),  # FIXME: support multi-column properties
              null=c.columns[0].nullable,
          ))
      return columns