from sqlalchemy.orm import ColumnProperty
from sqlalchemy.types import String
from sqlalchemy import event


class Error(Exception):
    """Base class for exceptions in this module."""
    pass


class ValidationError(Error):
    pass


class JeevesDBError(Error):
    """Exception raised for errors in the input.

    Attributes:
        expression -- input expression in which the error occurred
        message -- explanation of the error
    """

    def __init__(self, field, message):
        super().__init__(message)
        self.field = field
        self.message = message


def install_validator_listner(class_, key, inst):
    """Add validators for any attributes that can be validated."""
    prop = inst.prop
    # Only interested in simple columns, not relations
    if isinstance(prop, ColumnProperty) and len(prop.columns) == 1:
        col = prop.columns[0]
        # if we have string column with a length, create a length validator listner
        if isinstance(col.type, String) and col.type.length:
            event.listen(
                getattr(class_, key), 'set', LengthValidator(
                    col.name, col.type.length), retval=True)


class LengthValidator():
    def __init__(self, col_name, max_length):
        self.col_name = col_name
        self.max_length = max_length

    def __call__(self, state, value, oldvalue, initiator):
        if len(value) > self.max_length:
            raise ValidationError(
                "%s.%s: Length %d exceeds allowed %d" % (
                    state.__class__.__name__, state.__class__._map_columns(self.col_name),
                    len(value), self.max_length))
        return value