from django.db import models
from django.db.models.fields.related import ForwardManyToOneDescriptor
from django.utils.encoding import python_2_unicode_compatible
from django.utils.translation import get_language


class ArticleTranslationDescriptor(ForwardManyToOneDescriptor):
    """
    The set of articletranslation should not set any local fields.
    """
    def __set__(self, instance, value):
        if instance is None:
            raise AttributeError("%s must be accessed via instance" % self.field.name)
        setattr(instance, self.cache_name, value)
        if value is not None and not self.field.remote_field.multiple:
            setattr(value, self.field.related.get_cache_name(), instance)


class ColConstraint(object):
    # Anything with as_sql() method works in get_extra_restriction().
    def __init__(self, alias, col, value):
        self.alias, self.col, self.value = alias, col, value

    def as_sql(self, compiler, connection):
        qn = compiler.quote_name_unless_alias
        return '%s.%s = %%s' % (qn(self.alias), qn(self.col)), [self.value]


class ActiveTranslationField(models.ForeignObject):
    """
    This field will allow querying and fetching the currently active translation
    for Article from ArticleTranslation.
    """
    requires_unique_target = False

    def get_extra_restriction(self, where_class, alias, related_alias):
        return ColConstraint(alias, 'lang', get_language())

    def get_extra_descriptor_filter(self, instance):
        return {'lang': get_language()}

    def contribute_to_class(self, cls, name):
        super(ActiveTranslationField, self).contribute_to_class(cls, name)
        setattr(cls, self.name, ArticleTranslationDescriptor(self))


@python_2_unicode_compatible
class Article(models.Model):
    active_translation = ActiveTranslationField(
        'ArticleTranslation',
        from_fields=['id'],
        to_fields=['article'],
        related_name='+',
        on_delete=models.CASCADE,
        null=True,
    )
    pub_date = models.DateField()

    def __str__(self):
        try:
            return self.active_translation.title
        except ArticleTranslation.DoesNotExist:
            return '[No translation found]'


class NewsArticle(Article):
    pass


class ArticleTranslation(models.Model):
    article = models.ForeignKey(Article, models.CASCADE)
    lang = models.CharField(max_length=2)
    title = models.CharField(max_length=100)
    body = models.TextField()
    abstract = models.CharField(max_length=400, null=True)

    class Meta:
        unique_together = ('article', 'lang')
        ordering = ('active_translation__title',)


class ArticleTag(models.Model):
    article = models.ForeignKey(
        Article,
        models.CASCADE,
        related_name='tags',
        related_query_name='tag',
    )
    name = models.CharField(max_length=255)


class ArticleIdea(models.Model):
    articles = models.ManyToManyField(
        Article,
        related_name='ideas',
        related_query_name='idea_things',
    )
    name = models.CharField(max_length=255)