Fast and easy tree structures.

## Project description

Fast and easy tree structures.

In alpha, 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):

• less intrusive (no more inheriting issues due to Model, Manager & Queryset subclasses)

• easier to install

• easier to use

• more complete

• minimalist (less code, less database fields)

• bug-free

• safe (most of the logic is written directly in database)

• faster for all operations

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.

## Installation

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:

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:

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:

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:

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:

from django.db import models, migrations
from tree.operations import CreateTreeTrigger

class Migration(migrations.Migration):
dependencies = [
('tree', '0001_initial'),
]

operations = [
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.

## Usage

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:

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.get_roots()

#
# 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 there an error occurred.
pass

There is also a bunch of less useful lookups and transforms available. They will be documented with examples in the future.

## Differences with MPTT and treebeard

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.

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