code: def to_projection(self):
          
          roots = self._root_tables()
          if len(roots) > 1:
              raise com.RelationError(
                  'Cannot convert array expression '
                  'involving multiple base table references '
                  'to a projection'
              )
  
          table = TableExpr(roots[0])
          return table.projection([self])
docs: Promote this column expression to a table projection