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How do I order a model by parent category in django? – CentOS

I have a model “Category” with a ForeignKey to “parent_category”.
How can I order this model in the Django admin list view like:

- category 1
-- subcategory 1 of category 1
--- subsubcategory 1 of subcategory 1 of category 1
-- subcategory 2 of category 1
-- subcategory 3 of category 1
- category 2
-- subcategory 1 of category 2
-- subcategory 2 of category 2

I have tried the following but this won’t work. So I need some help to order the function ‘get_relative_name’.

class PrivateContentCategory(models.Model):
    name = models.CharField(
        max_length=250,
        verbose_name=_('Naam'),
    )
    slug = models.SlugField(
        verbose_name=_('Url'),
        blank=True,
    )
    parent_category = models.ForeignKey(
        'self',
        on_delete=models.SET_NULL,
        related_name='child_category_list',
        verbose_name=_('Hoofdcategorie'),
        blank=True,
        null=True,
    )

    def __str__(self):
        str = self.name
        parent_category_obj = self.parent_category
        while parent_category_obj is not None:
            str = parent_category_obj.name + ' --> ' + str
            parent_category_obj = parent_category_obj.parent_category
        return str

    def get_relative_name(self):
        str = self.name
        parent_category_obj = self.parent_category
        while parent_category_obj is not None:
            str = '--' + str
            parent_category_obj = parent_category_obj.parent_category
    get_relative_name.short_description = _('Naam')
    get_relative_name.admin_order_field = [
        'parent_category__parent_category',
        'name',
    ]

EDIT!!!
The names of the parent category should not be displayed with the category. I had written it like this to display how the model should be ordered. The display of the list will just be:

- OS
-- Windows
--- Windows 7
--- Windows 8
--- Windows 10
-- Mac
-- Linux
--- Debian
---- Ubuntu
--- Fedora
---- CentOS
---- Oracle Linux
Tags:

3

Answers


  1. Chosen as BEST ANSWER
    0

    What worked for me was to add a new field "absolute_name" to the model which will be auto populated with a pre_save signal. After an instance is saved, this field will conain the names for all parent_categories of the instance before it own name. At last, I just needed to order the instance on this field:

    class PrivateContentCategory(models.Model):
    name = models.CharField(
        max_length=250,
        verbose_name=_('Naam'),
    )
    slug = models.SlugField(
        verbose_name=_('Url'),
        blank=True,
    )
    parent_category = models.ForeignKey(
        'self',
        on_delete=models.SET_NULL,
        related_name='child_category_list',
        verbose_name=_('Hoofdcategorie'),
        blank=True,
        null=True,
    )
    absolute_name = models.TextField(
        verbose_name=_('Absolute naam'),
        blank=True,
    )

    def __str__(self):
        return self.absolute_name

    def get_relative_name(self):
        str = self.name
        parent_category_obj = self.parent_category
        while parent_category_obj is not None:
            str = '--' + str
            parent_category_obj = parent_category_obj.parent_category
        return str
    get_relative_name.short_description = _('Naam')
    get_relative_name.admin_order_field = [
        'absolute_name',
    ]

    class Meta:
        verbose_name = _('Privé inhoud categorie')
        verbose_name_plural = _('Privé inhoud categorieën')
        ordering = [
            'absolute_name',
        ]


@receiver(models.signals.pre_save, sender=PrivateContentCategory)
def pre_save_private_content_category_obj(sender, instance, **kwargs):
    # START Generate instance.absolute_name
    instance.absolute_name = instance.name
    parent_category_obj = instance.parent_category
    while parent_category_obj is not None:
        instance.absolute_name = parent_category_obj.name + ' --> ' + instance.absolute_name
        parent_category_obj = parent_category_obj.parent_category
    # END Generate instance.absolute_name

  2. 0

    In order to be able to order by it, you need to annotate the queryset in the modeladmin, so a method on the model won’t help.

    admin.py

    from django.db.models.expressions import F
...


@admin.register(PrivateContentCategory)
class PrivateContentCategoryAdmin(admin.ModelAdmin):
    list_display = (
        'name',
        'relative_name',
    )

    def get_queryset(self, request):
        qs = super().get_queryset(request)  # type: QuerySet
        qs = qs.annotate(relative_name=F('name'))  # for now :)
        return qs

    def relative_name(self, obj: PrivateContentCategory):
        return obj.relative_name

    relative_name.admin_order_field = 'relative_name'

    This will add a column to the admin, and allow you to click-sort it.

    One thing this will not allow you to do it to have a default sort on that column. This will fail:

    class PrivateContentCategoryAdmin(admin.ModelAdmin):
   ...
   ordering = ('relative_name',)

    ERRORS:
    <class ‘cats.admin.PrivateContentCategoryAdmin’>: (admin.E033) The value of ‘ordering[0]’ refers to ‘relative_name’, which is not an attribute of ‘cats.PrivateContentCategory’.

    This is a long-standing bug in Django: https://code.djangoproject.com/ticket/17522
    There are ways around it, but I am getting off topic…

    So the second problem is, obviously, we need to construct the relative names there, instead of that F('name'). I could be wrong, but I think the only DB engine that supports this on-the-fly is Postgres. If you are using a different DB engine, then I guess you are left with having to denormalize your data a bit and have a column with full parent name on every child.

    There could be better ways to do this, but here is how I have done it:

    admin.py

    ...
from django.db.models.expressions import RawSQL


relative_name_query = '''
    WITH RECURSIVE "relative_names" as (
        SELECT "id", "parent_category_id", CAST("name" AS TEXT)
        FROM "{table}"
        WHERE "parent_category_id" IS NULL
        UNION ALL
        SELECT "t"."id", "t"."parent_category_id", CONCAT_WS('/', "r"."name", "t"."name")
        FROM "{table}" "t"
        JOIN "relative_names" "r" ON "t"."parent_category_id" = "r"."id"
    )
    SELECT "name"
    FROM "relative_names" WHERE "relative_names"."id" = "{table}"."id"
'''


@admin.register(PrivateContentCategory)
class PrivateContentCategoryAdmin(admin.ModelAdmin):
        ...

        # instead of that F('name') line:
        qs = qs.annotate(relative_name=RawSQL(
            relative_name_query.format(
                table=qs.model._meta.db_table,
            ),
            (),
        ))

    P.S.

    Oracle seems to support it as well, though with a different syntax: SQL recursive query on self referencing table (Oracle)

    P.P.S.

    If you end up having to keep parent name on the model, then the annotate looks something like this:

    qs = qs.annotate(relative_name=Concat(F('parent_name'), Value('/'), F('name')))

    P.P.P.S.

    It is possible to add two annotations, one for displaying values and another for sorting. Actually, looking at your question again, I think this will be needed because your example has subcat -- cat and not cat -- subcat as I assumed above. For this we need two annotations, one of which will be returned from relative_name modeladmin method and the other will be for relative_name.admin_order_field.

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  3. 0

    A much cleaner, more efficient solution is to use django-mptt:

    from mptt.models import MPTTModel
from mptt.fields import TreeForeignKey

class PrivateContentCategory(MPTTModel):
    name = models.CharField(max_length=250)
    slug = models.SlugField(blank=True)
    parent_category = TreeForeignKey(
        'self',
        on_delete=models.SET_NULL,
        related_name='child_category_list',
        blank=True,
        null=True,
    )

    class MPTTMeta:
        order_insertion_by = ['name']

    If you want to generate a <select> dropdown in forms using this model:

    from mptt.forms import TreeNodeMultipleChoiceField

class SomeForm(forms.Form):
    category = TreeNodeMultipleChoiceField(
        queryset = PrivateContentCategory.objects.all()
    )

    This also works in the admin:

    from mptt.admin import MPTTModelAdmin

class PrivateContentCategoryAdmin(MPTTModelAdmin):
    mptt_level_indent = 20

admin.site.register(PrivateContentCategory, PrivateContentCategoryAdmin)
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