Fast and easy tree structures.
.. image:: http://img.shields.io/pypi/v/django-tree.svg?style=flat-square :target: https://pypi.python.org/pypi/django-tree
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In beta, it can’t be used yet in production.
This tool works in a very similar way to django-mptt and django-treebeard, however it’s so different in conception that it was better and faster to start from scratch than to rewrite the existing solutions.
Compared to these solutions, django-tree aims to have these advantages (some of them are already there):
However, there is nothing groundbreaking here: this is only the result of a proper use of the latest Django improvements, combined with a good knowledge of SQL.
The detailed benchmark <benchmark/results/results.rst>_ gives a good idea
on how well django-tree performs compared to other Django solutions.
All that while being simpler to use, more robust and fully generalized to raw SQL, bulk etc.
A few noteworthy extracts of the benchmark (less is better):
.. image:: benchmark/results/postgresql_-_Create_allobjects.svg .. image:: benchmark/results/postgresql-_Rebuildpaths.svg .. image:: benchmark/results/postgresql-Create[root].svg
Django-tree requires Django 1.8, 1.11 or 2.0 and Python 2 or 3. For the moment, django-tree is only for PostgreSQL. It will be adapted in the future for other databases.
After installing the module, you need to add 'tree', to your
INSTALLED_APPS, then add a PathField to a model with a
ForeignKey('self'), typically named parent (use the parent_field
argument of CreateTreeTrigger if the field has another name).
PathField stores Path objects which have methods to execute queries,
such as getting all the descendants of the current object, its siblings, etc.
To call these methods more conveniently, you can add TreeModelMixin
to your model. The inheriting order is not important, as the mixin methods
do not clash with Django. If you have multiple PathField
on the same model, you will have to specify the field name in the method
you’re calling using path_field.
This should give you a model like this:
.. code:: python
from django.db.models import Model, CharField, ForeignKey, BooleanField
from tree.fields import PathField
from tree.models import TreeModelMixin
class YourModel(Model, TreeModelMixin):
name = CharField(max_length=30)
parent = ForeignKey('self', null=True, blank=True)
path = PathField()
public = BooleanField(default=False)
class Meta:
ordering = ['path']Then you need to create the SQL trigger that will automatically update path.
To do that, create a migration with a dependency
to the latest django-tree migration and add a CreateTreeTrigger operation:
.. code:: python
from django.db import migrations
from tree.operations import CreateTreeTrigger
class Migration(migrations.Migration):
dependencies = [
('tree', '0001_initial'),
]
operations = [
CreateTreeTrigger('your_app.YourModel'),
]If you already have data in YourModel, you will need to add an operation
for allowing SQL NULL values before creating the trigger,
then rebuild the paths and revert the allowance of NULL values:
.. code:: python
from django.db import migrations
from tree.fields import PathField
from tree.operations import CreateTreeTrigger, RebuildPaths
class Migration(migrations.Migration):
dependencies = [
('tree', '0001_initial'),
]
operations = [
migrations.AlterField('YourModel', 'path', PathField(null=True)),
CreateTreeTrigger('YourModel'),
RebuildPaths('YourModel', 'path'),
migrations.AlterField('YourModel', 'path', PathField()),
]However, the model above is not ordered. The children of a same parent will be
ordered by primary key. You can specify how children are ordered using the
order_by argument of PathField. If needed, you can add a field
for users to explicitly order these objects, typically a position field.
Example model:
.. code:: python
from django.db.models import (
Model, CharField, ForeignKey, IntegerField, BooleanField)
from tree.fields import PathField
from tree.models import TreeModelMixin
class YourModel(Model, TreeModelMixin):
name = CharField(max_length=30)
parent = ForeignKey('self', null=True, blank=True)
position = IntegerField(default=1)
path = PathField(order_by=['position', 'name'])
public = BooleanField(default=False)
class Meta:
ordering = ['path']And the corresponding migration:
.. code:: python
from django.db import models, migrations
from tree.operations import CreateTreeTrigger
class Migration(migrations.Migration):
dependencies = [
('tree', '0001_initial'),
]
operations = [
migrations.AddField('YourModel', 'position',
models.IntegerField(default=1))
CreateTreeTrigger('YourModel'),
]Here, the children of a same parent will be ordered by position, and then by name if the position is the same.
.. note::
You can also use ``PathField`` without adding a ``CreateTreeTrigger``
operation. However, the field will not automatically be updated, you
will have to do it by yourself. In most cases this is not useful, so you
should not use ``PathField`` without ``CreateTreeTrigger`` unless you know
what you are doing.PathField is automatically filled thanks to CreateTreeTrigger,
you don’t need to set, modify, or even see its value once it is installed.
But you can use the Path object it stores or the more convenient
TreeModelMixin to get tree information about the current instance,
or make complex queries on the whole tree structure.
Example to show you most of the possibilities:
.. code:: python
obj = YourModel.objects.all()[0]
obj.path.get_level()
obj.get_level() # Shortcut for the previous method, if you use
# `TreeModelMixin`. Same for other object methods below.
obj.is_root()
obj.is_leaf()
obj.get_children()
obj.get_children().filter(public=True)
obj.get_ancestors()
obj.get_ancestors(include_self=True)
obj.get_descendants(include_self=True)
obj.get_siblings()
obj.get_prev_sibling() # Fetches the previous sibling.
obj.get_next_sibling()
# Same as `get_prev_sibling`, except that we get the first public one.
obj.get_prev_siblings().filter(public=True).first()
other = YourModel.objects.all()[1]
obj.is_ancestor_of(other)
obj.is_descendant_of(other, include_self=True)
YourModel.objects.filter_roots()
#
# Advanced usage
# Use the following methods only if you understand exactly what they mean.
#
YourModel.rebuild_paths() # Rebuilds all paths of this field, useful only
# if something is broken, which shouldn’t happen.
YourModel.disable_tree_trigger() # Disables the SQL trigger.
YourModel.enable_tree_trigger() # Restores the SQL trigger.
with YourModel.disabled_tree_trigger():
# What happens inside this context manager is ignored
# by the SQL trigger.
# The trigger is restored after that, even if an error occurred.
passThere is also a bunch of less useful lookups and transforms available. They will be documented with examples in the future.
Level vs depth ..............
django-mptt and django-treebeard use two different names to designate almost the same thing: MPTT uses level and treebeard uses depth. Both are integers to show how much distant is a node from the top of the tree. The only difference is that level should start by convention with 1 and depth should start with 0.
Unfortunately, both MPTT and treebeard are wrong about the indexing: MPTT starts its level with 0 and treebeard starts its depth with 1.
Django-tree finally fixes this issue by implementing a level starting by 1, and no depth to avoid confusion. One name had to be chosen, and I find that “level” represents more accurately the idea that we deal with an abstract tree, where all the node of the same level are on the same row. In comparison, “depth” sounds like we’re actually digging a real root, and it gives the impression that a child of a root can be at a different depth than a child of another root, like in real life.