'''
Fields.

Contains two custom fields:
    * :class:`EavSlugField`
    * :class:`EavDatatypeField`
'''

import re

from django.core.exceptions import ValidationError
from django.db import models
from django.utils.translation import ugettext_lazy as _


class EavSlugField(models.SlugField):
    '''
    The slug field used by :class:`~eav.models.Attribute`
    '''

    def validate(self, value, instance):
        '''
        Slugs are used to convert the Python attribute name to a database
        lookup and vice versa. We need it to be a valid Python identifier.
        We don't want it to start with a '_', underscore will be used
        var variables we don't want to be saved in db.
        '''
        super(EavSlugField, self).validate(value, instance)
        slug_regex = r'[a-z][a-z0-9_]*'
        if not re.match(slug_regex, value):
            raise ValidationError(_(u"Must be all lower case, " \
                                    u"start with a letter, and contain " \
                                    u"only letters, numbers, or underscores."))

    @staticmethod
    def create_slug_from_name(name):
        '''
        Creates a slug based on the name
        '''
        name = name.strip().lower()

        # Change spaces to underscores
        name = '_'.join(name.split())

        # Remove non alphanumeric characters
        return re.sub('[^\w]', '', name)


class EavDatatypeField(models.CharField):
    '''
    The datatype field used by :class:`~eav.models.Attribute`
    '''

    def validate(self, value, instance):
        '''
        Raise ``ValidationError`` if they try to change the datatype of an
        :class:`~eav.models.Attribute` that is already used by
        :class:`~eav.models.Value` objects.
        '''
        super(EavDatatypeField, self).validate(value, instance)
        if not instance.pk:
            return
        if type(instance).objects.get(pk=instance.pk).datatype == instance.datatype:
            return
        if instance.value_set.count():
            raise ValidationError(_(u"You cannot change the datatype of an "
                                    u"attribute that is already in use."))