Create custom template tag in Django?

Question

I have next models in my Django project.

models.py:

class Document(models.Model):
    name = models.TextField(blank=True, null=True)

    def get_children(self):
        return [dc.child for dc in DocumentClosure.objects.filter(parent=self)]

class DocumentClosure(models.Model):
    parent = models.ForeignKey(
        Document,
        on_delete=models.CASCADE,
        related_name="closures_as_parent",
        db_column='parent_id',
        blank=True,
        null=True
    )

    child = models.ForeignKey(
        Document,
        on_delete=models.CASCADE,
        related_name="closures_as_child",
        db_column='child_id',
        blank=True,
        null=True
    )

    level = models.IntegerField(
        default=0,
        blank=True,
        null=True
    )

Next models create 2 table in database with "Closure Table" architecture. DocumentClosure store information about ancestors and descendants. I want to show tree in template as it make django-mptt application.

I started with next code. Now I need to rewrite get_children() method based on my current models. Can someone help me with this method?!

template:

<ul>
    {% recursetree documents %}
        <li>
            {{ node.name }}
            <ul class="children">
                {{ children }}
            </ul>
        </li>
    {% endrecursetree %}
</ul>

custom_tag.py:

from django import template

register = template.Library()

@register.tag
def recursetree(parser, token):
    bits = token.contents.split()
    if len(bits) != 2:
        raise template.TemplateSyntaxError(_('%s tag requires a queryset') % bits[0])
    queryset_var = template.Variable(bits[1])
    template_nodes = parser.parse(('endrecursetree',))
    parser.delete_first_token()
    return RecurseTreeNode(template_nodes, queryset_var)


class RecurseTreeNode(template.Node):
    def __init__(self, template_nodes, queryset_var):
        self.template_nodes = template_nodes
        self.queryset_var = queryset_var

    def _render_node(self, context, node):
        bits = []
        context.push()
        for child in node.get_children():  # get_children() initialized in models.py file | rewrite this method
            bits.append(self._render_node(context, child))
        context['node'] = node
        context['children'] = mark_safe(''.join(bits))
        rendered = self.template_nodes.render(context)
        context.pop()
        return rendered

    def render(self, context):
        queryset = self.queryset_var.resolve(context)
        roots = queryset
        bits = [self._render_node(context, node) for node in roots]
        return ''.join(bits)

EDIT: After python manage.py runserver command in console I see the endless repetitive queries to the database. But after some time it raise error:

Traceback (most recent call last):
File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/core/handlers/base.py", line 217, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/core/handlers/base.py", line 215, in _get_response
    response = response.render()
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/response.py", line 107, in render
    self.content = self.rendered_content
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/response.py", line 84, in rendered_content
    content = template.render(context, self._request)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/backends/django.py", line 66, in render
    return self.template.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 207, in render
    return self._render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 199, in _render
    return self.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/loader_tags.py", line 177, in render
    return compiled_parent._render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 199, in _render
    return self.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/loader_tags.py", line 177, in render
    return compiled_parent._render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 199, in _render
    return self.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/loader_tags.py", line 72, in render
    result = block.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/loader_tags.py", line 72, in render
    result = block.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/loader_tags.py", line 216, in render
    return template.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 209, in render
    return self._render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 199, in _render
    return self.nodelist.render(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 990, in render
    bit = node.render_annotated(context)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/template/base.py", line 957, in render_annotated
    return self.render(context)
  File "/Applications/Projects/web/project/documents/templatetags/documents_tags.py", line 41, in render
    bits = [self._render_node(context, node) for node in roots]
  File "/Applications/Projects/web/project/documents/templatetags/documents_tags.py", line 31, in _render_node
    bits.append(self._render_node(context, child))
File "/Applications/Projects/web/project/documents/models.py", line 117, in get_children
    return [dc.child for dc in DocumentClosure.objects.filter(parent=self)]
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/query.py", line 250, in __iter__
    self._fetch_all()
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/query.py", line 1118, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 871, in execute_sql
    sql, params = self.as_sql()
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/backends/oracle/compiler.py", line 21, in as_sql
    with_col_aliases=with_col_aliases,
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 436, in as_sql
    where, w_params = self.compile(self.where) if self.where is not None else ("", [])
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 373, in compile
    sql, params = node.as_sql(self, self.connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/sql/where.py", line 79, in as_sql
    sql, params = compiler.compile(child)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 373, in compile
    sql, params = node.as_sql(self, self.connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/fields/related_lookups.py", line 127, in as_sql
    return super(RelatedLookupMixin, self).as_sql(compiler, connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/lookups.py", line 169, in as_sql
    lhs_sql, params = self.process_lhs(compiler, connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/lookups.py", line 162, in process_lhs
    db_type = self.lhs.output_field.db_type(connection=connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/fields/related.py", line 991, in db_type
    return self.target_field.rel_db_type(connection=connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 951, in rel_db_type
    return IntegerField().db_type(connection=connection)
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 166, in __init__
    if isinstance(choices, collections.Iterator):
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/bin/../lib/python2.7/abc.py", line 141, in __instancecheck__
    subtype in cls._abc_negative_cache):
  File "/Users/nurzhan_nogerbek/Virtualenvs/py2714/bin/../lib/python2.7/_weakrefset.py", line 75, in __contains__
    return wr in self.data
RuntimeError: maximum recursion depth exceeded in cmp

Lets say I have such tree. Example:

A
|
 - B
   |
    - C

document table:

id | parent_id | name
1  |           | A
2  | 1         | B
3  | 2         | C

Finally document_closure table:

id | parent_id | child_id | level
1  | 1         | 1        | 0
2  | 2         | 2        | 0
3  | 3         | 3        | 0
4  | 1         | 2        | 1
5  | 2         | 3        | 1
6  | 1         | 3        | 2

Answer 1

Select all DocumentClosure objects that have your Document as parent and return their children.

Return a list with their linked child Document like this:

class Document(models.Model):
    name = models.TextField(blank=True, null=True)

    def get_children(self):
        return [dc.child for dc in DocumentClosure.objects.filter(parent=self)]

You should consider renaming the related_name of both ForeignKey relations on DocumentClosure - they create properties with confusing names. Document.parents returns a Queryset of DocumentClosure objects where the given Document is the parent. One would expect it to include the parents of the current document. Maybe something like closures_as_parent and closures_as_child fits better.