django/tests/foreign_object/models/empty_join.py

from django.db import models
from django.db.models.fields.related import (
    ForeignObjectRel, ReverseManyToOneDescriptor,
)
from django.db.models.lookups import StartsWith
from django.db.models.query_utils import PathInfo


class CustomForeignObjectRel(ForeignObjectRel):
    """
    Define some extra Field methods so this Rel acts more like a Field, which
    lets us use ReverseManyToOneDescriptor in both directions.
    """
    @property
    def foreign_related_fields(self):
        return tuple(lhs_field for lhs_field, rhs_field in self.field.related_fields)

    def get_attname(self):
        return self.name


class StartsWithRelation(models.ForeignObject):
    """
    A ForeignObject that uses StartsWith operator in its joins instead of
    the default equality operator. This is logically a many-to-many relation
    and creates a ReverseManyToOneDescriptor in both directions.
    """
    auto_created = False

    many_to_many = False
    many_to_one = True
    one_to_many = False
    one_to_one = False

    rel_class = CustomForeignObjectRel

    def __init__(self, *args, **kwargs):
        kwargs['on_delete'] = models.DO_NOTHING
        super(StartsWithRelation, self).__init__(*args, **kwargs)

    @property
    def field(self):
        """
        Makes ReverseManyToOneDescriptor work in both directions.
        """
        return self.remote_field

    def get_extra_restriction(self, where_class, alias, related_alias):
        to_field = self.remote_field.model._meta.get_field(self.to_fields[0])
        from_field = self.model._meta.get_field(self.from_fields[0])
        return StartsWith(to_field.get_col(alias), from_field.get_col(related_alias))

    def get_joining_columns(self, reverse_join=False):
        return tuple()

    def get_path_info(self):
        to_opts = self.remote_field.model._meta
        from_opts = self.model._meta
        return [PathInfo(from_opts, to_opts, (to_opts.pk,), self, False, False)]

    def get_reverse_path_info(self):
        to_opts = self.model._meta
        from_opts = self.remote_field.model._meta
        return [PathInfo(from_opts, to_opts, (to_opts.pk,), self.remote_field, False, False)]

    def contribute_to_class(self, cls, name, private_only=False):
        super(StartsWithRelation, self).contribute_to_class(cls, name, private_only)
        setattr(cls, self.name, ReverseManyToOneDescriptor(self))


class BrokenContainsRelation(StartsWithRelation):
    """
    This model is designed to yield no join conditions and
    raise an exception in ``Join.as_sql()``.
    """
    def get_extra_restriction(self, where_class, alias, related_alias):
        return None


class SlugPage(models.Model):
    slug = models.CharField(max_length=20, unique=True)
    descendants = StartsWithRelation(
        'self',
        from_fields=['slug'],
        to_fields=['slug'],
        related_name='ascendants',
    )
    containers = BrokenContainsRelation(
        'self',
        from_fields=['slug'],
        to_fields=['slug'],
    )

    class Meta:
        ordering = ['slug']

    def __str__(self):
        return 'SlugPage %s' % self.slug