Allow generic date views to use related fields as date_field

[Lefteris Nikoltsios]

For example if we have some models like that:

class Blog(models.Model):
    title = models.TextField()
    pubdate = models.DateField()

class Article(models.Model):
    blog = models.ForeignKey(Blog)
    title = models.TextFIeld()
    pubdate = models.DateField()

the following view:

from django.views.generic.dates import MonthArchiveView

from .models import Article


class ArticleMonthArchiveView(MonthArchiveView):
    queryset = Article.objects.all()
    date_field = 'blog__pub_date'

will raise FieldDoesNotExist.

[Lefteris Nikoltsios]

​PR