from django.db import models

from django.db.models.fields.related import ForwardManyToOneDescriptor

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)

        self.field.set_cached_value(instance, value)

        if value is not None and not self.field.remote_field.multiple:

            self.field.remote_field.set_cached_value(value, instance)

class ColConstraint:

    # 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, 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().contribute_to_class(cls, name)

        setattr(cls, self.name, ArticleTranslationDescriptor(self))

class ActiveTranslationFieldWithQ(ActiveTranslationField):

    def get_extra_descriptor_filter(self, instance):

        return models.Q(lang=get_language())

class Article(models.Model):

    active_translation = ActiveTranslationField(

        "ArticleTranslation",

        from_fields=["id"],

        to_fields=["article"],

        related_name="+",

        on_delete=models.CASCADE,

        null=True,

    )

    active_translation_q = ActiveTranslationFieldWithQ(

        "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.TextField(null=True)

    class Meta:

        unique_together = ("article", "lang")

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)