def to_projection(self):
"""
Promote this column expression to a table projection
"""
roots = self._root_tables()
if len(roots) > 1:
raise RelationError('Cannot convert array expression involving '
'multiple base table references to a '
'projection')
table = TableExpr(roots[0])
return table.projection([self])
def assert_valid(self, expr):
if not self.validate(expr):
msg = self._error_message(expr)
raise RelationError(msg)
