Represent a tree of objects in Django template

Question

I have a Django model which has a ForeignKey to the same class, effectively making a tree:

class Tag(models.Model):
    name = models.CharField(max_length=50)
    parent = models.ForeignKey('self', blank=True, null=True)

Playing around with a recursive in the Django shell (./manage.py shell), I am easily able to represent the tree as plain text:

def nodes(parent, level):
    children = Tag.objects.filter(parent=parent)
    for c in children:
        spaces = ""
        for i in xrange(0,level):
            spaces+="  "
        print "%s%s" % (spaces,c.name)
        nodes(c.pk,level+1)

nodes(None,0)

What I am unsure of is how to get the entire tree into a Django template. I've created a custom template tag to make this easier, but I can't figure out how to pass the data to the template to easily iterate over the tree for display in a template. Here's the basic template tag.

@register.inclusion_tag("core/tags.html")
def render_tags(**kwargs):
    tags = Tag.objects.all()
    return {"tags":tags}

I know the above is very basic, I just am not sure where to go from here. I thought it might be easier if the Tag class had a function to get its children, so I also have on the class:

    def children(self):
        return Tag.objects.filter(parent=self.pk)

I use self.pk there, then the root of the tree is simply rootTag=Tag(), since it has no pk since it is not saved, rootTag.children() will find any Tags which do not have a parent Tag, and any of these tags can then just continue to have their children() function called. But like I said, I do not know how to turn this into a single data structure of some sort to pass to my template.

Thoughts? I think I probably want to build a kind of dict, I'm just not able to follow through here.

Answer 1

As Jproffitt mentionned it Django MPTT is a nice way to achieve what you want.

Using it, you can then access children instance, recursively, in your template, like this:

{% load mptt_tags %}
<h1>Tags</h1>
<ul>
{% recursetree tags %}
    <li>{{ node.name }}
        {% if not node.is_leaf_node %}
            <ul>
                {{ children }}
            </ul>
        {% endif %}
    </li>
{% endrecursetree %}
</ul>

I have it for one of my projects and it's easy to set up and to use.

Alternative solution without a third-party app

If you don't want to use a dedicated app, I think these solution might work (untested):

# yourapp/tags_list.html
<ul>
{% for tag in tags %}
    <li>{{ tag.name }}</li>
    {% if tag.children.exists %}
       {% with tag.children.all as tags %}
            {% include "yourapp/tags_list.html" %}
        {% endwith %}    
    {% endif %}
{% endfor %}
</ul>

This way, the template should just call himself recursively until there is no more children tags. This solution needs that you specify a related name in your Tag model :

parent = models.ForeignKey('self', blank=True, null=True, related_name="children")

However, be careful, these solution will involve more database queries than using MPTT.

Answer 2

The more I searched around the more I realized trying to do this with template recursion isn't the way to go, but I'd still like to avoid django-mptt for now because it just seems like too much for what I feel is a simple problem, and I don't plan on having very large trees.

An answer to this related question noted that template recursion really shouldn't be used and to instead handle serialization in the view. I had thought this would be better as I mentioned in the original question, assuming I needed to build a dict most likely.

This answer is basically what I was looking for. The serializable_object is something I can pass into a template, and if I decide I want to go with json later on it's quite easy to do as noted in that answer.

I am going to select Eliot's answer as correct since django-mptt is probably the "most correct" thing to do here, but this is an alternate solution for anyone looking to avoid another third party app.

class Tag(models.Model):
    name = models.CharField(max_length=50)
    description = models.CharField(max_length=100, blank=True)
    parent = models.ForeignKey('self', blank=True, null=True)

    # Not necessary, can use Tag.tag_set.all()
    # But this function uses pk which allows
    #    rootTag=Tag() 
    #    rootTag.children()
    # Because rootTag has no pk
    def children(self):
        return Tag.objects.filter(parent=self.pk)

    def serializable_object(self):
        obj = {'name': self.name, 'children': []}
        for child in self.children():
            obj['children'].append(child.serializable_object())
        return obj

With that model, you can either do:

rootTag=Tag()
rootTag.serializable_object()

Or:

tags = []
for t in Tag.objects.filter(parent=None):
    tags.append(t.serializable_object())

# And if you need JSON, 
from django.utils import simplejson
simplejson.dumps(tags)