models.rst

Models and Managers

Setting up a Django model for MPTT

Start with a basic subclass of MPTTModel, something like this:

from django.db import models
from mptt.models import MPTTModel, TreeForeignKey

class Genre(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children')

You must define a parent field which is a ForeignKey to 'self'. Recommended: use TreeForeignKey. You can call it something different if you want - see Model Options below.

Because you're inheriting from MPTTModel, your model will also have a number of other fields: level, lft, rght, and tree_id. Most of the time you won't need to use these fields directly, but it's helpful to know they're there.

Model Options

Sometimes you might want to change the names of the above fields, for instance if you've already got a field named level and you want to avoid conflicts.

To change the names, create an MPTTMeta class inside your class:

class Genre(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children')

    class MPTTMeta:
        level_attr = 'mptt_level'
        order_insertion_by=['name']

The available options for the MPTTMeta class are:

parent_attr

The name of a field which relates the model back to itself such that each instance can be a child of another instance. Defaults to 'parent'.

Users are responsible for setting this field up on the model class, which can be done like so:

parent = TreeForeignKey('self', null=True, blank=True, related_name='children')

For the following four arguments, if fields with the given names do not exist, they will be added to the model dynamically:

left_attr

The name of a field which contains the left tree node edge indicator, which should be a PositiveIntegerField. Defaults to 'lft'.

right_attr

The name of a field which contains the right tree node edge indicator, which should be a PositiveIntegerField. Defaults to 'rght'.

tree_id_attr

The name of a field which contains the tree id of each node, which should be a PositiveIntegerField. Defaults to 'tree_id'.

Items which do not have a parent are considered to be "root" nodes in the tree and will be allocated a new tree id. All descendants of root nodes will be given the same tree id as their root node.

level_attr

The name of a field which contains the (zero-based) level at which an item sits in the tree, which should be a PositiveIntegerField. Defaults to 'level'.

For example, root nodes would have a level of 0 and their immediate children would have have a level of 1.

tree_manager_attr

The name of an attribute which will hold a custom manager which is used to work with trees of model instances. Defaults to 'tree'.

order_insertion_by

A list of field names which should define ordering when new tree nodes are being inserted or existing nodes are being reparented, with the most significant ordering field name first. Defaults to [].

It is assumed that any field identified as defining ordering will never be NULL in the database.

Note that this will require an extra database query to determine where nodes should be positioned when they are being saved. This option is handy if you're maintaining mostly static structures, such as trees of categories, which should always be in alphabetical order.

Registration of existing models

The preferred way to do model registration in django-mptt 0.4 is by subclassing MPTTModel.

However, sometimes that doesn't quite work. For instance, suppose you want to modify Django's Group model to be hierarchical.

You can't subclass MPTTModel without modifying the Group source. Instead, you can do:

import mptt
from mptt.fields import TreeForeignKey
from django.contrib.auth.models import Group

# add a parent foreign key
TreeForeignKey(Group, blank=True, null=True).contribute_to_class(Group, 'parent')

mptt.register(Group, order_insertion_by=['name'])

mptt.register() was removed in 0.4.0 but restored in 0.4.2, when this use case was reported.

MPTTModel instance methods

Subclasses of MPTTModel have the following instance methods:

get_ancestors(ascending=False)

creates a QuerySet containing the ancestors of the model instance.

These default to being in descending order (root ancestor first, immediate parent last); passing True for the ascending argument will reverse the ordering (immediate parent first, root ancestor last).

get_children()

Creates a QuerySet containing the immediate children of the model instance, in tree order.

The benefit of using this method over the reverse relation provided by the ORM to the instance's children is that a database query can be avoided in the case where the instance is a leaf node (it has no children).

get_descendants(include_self=False)

Creates a QuerySet containing descendants of the model instance, in tree order.

If include_self is True, the QuerySet will also include the model instance itself.

get_descendant_count()

Returns the number of descendants the model instance has, based on its left and right tree node edge indicators. As such, this does not incur any database access.

get_next_sibling()

Returns the model instance's next sibling in the tree, or None if it doesn't have a next sibling.

get_previous_sibling()

Returns the model instance's previous sibling in the tree, or None if it doesn't have a previous sibling.

get_root()

Returns the root node of the model instance's tree.

get_siblings(include_self=False)

Creates a QuerySet containing siblings of the model instance. Root nodes are considered to be siblings of other root nodes.

If include_self is True, the QuerySet will also include the model instance itself.

insert_at(target, position='first-child', save=False)

Positions the model instance (which must not yet have been inserted into the database) in the tree based on target and position (when appropriate).

If save is True, the model instance's save() method will also be called.

is_child_node()

Returns True if the model instance is a child node, False otherwise.

is_leaf_node()

Returns True if the model instance is a leaf node (it has no children), False otherwise.

is_root_node()

Returns True if the model instance is a root node, False otherwise.

move_to(target, position='first-child')

Moves the model instance elsewhere in the tree based on target and position (when appropriate).

Note

It is assumed that when you call this method, the tree fields in the instance you've called it on, and in any target instance passed in, reflect the current state of the database.

Modifying the tree fields manually before calling this method or using tree fields which are out of sync with the database can result in the tree structure being put into an inaccurate state.

If target is another model instance, it will be used to determine the type of movement which needs to take place, and will be used as the basis for positioning the model when it is moved, in combination with the position argument.

A target of None indicates that the model instance should be turned into a root node. The position argument is disregarded in this case.

Valid values for the position argument and their effects on movement are:

'first-child'

The instance being moved should have target set as its new parent and be placed as its first child in the tree structure.

'last-child'

The instance being moved should have target set as its new parent and be placed as its last child in the tree structure.

'left'

The instance being moved should have target's parent set as its new parent and should be placed directly before target in the tree structure.

'right'

The instance being moved should have target's parent set as its new parent and should be placed directly after target in the tree structure.

A ValueError will be raised if an invalid value is given for the position argument.

Note that some of the moves you could attempt to make with this method are invalid - for example, trying to make an instance be its own child or the child of one of its descendants. In these cases, a mptt.exceptions.InvalidMove exception will be raised.

The instance itself will be also modified as a result of this call, to reflect the state of its updated tree fields in the database, so it's safe to go on to save it or use its tree fields after you've called this method.

TreeForeignKey

0.5

It's recommended you use mptt.fields.TreeForeignKey wherever you have a foreign key to an MPTT model. This includes the parent link you've just created on your model.

TreeForeignKey is just like a regular ForeignKey but it makes the default form field display choices in tree form.

The TreeManager custom manager

A custom manager, TreeManager is also added to your MPTTModel subclasses. The attribute this manager can be accessed through is specified by the tree_manager_attr option (default 'tree')

Any QuerySet created with this manager will be ordered based on the tree structure, with root nodes appearing in tree id order and and their descendants being ordered in a depth-first fashion.

Methods

The following manager methods are available:

add_related_count(queryset, rel_cls, rel_field, count_attr, cumulative=False)

Adds a related item count to a given QuerySet using its extra method, for a model which has a relation to this manager's model.

rel_cls

A Django model class which has a relation to this manager's model.

rel_field

The name of the field in rel_cls which holds the relation.

count_attr

The name of an attribute which should be added to each item in this QuerySet, containing a count of how many instances of rel_cls are related to it through rel_field.

cumulative

If True, the count will be for each item and all of its descendants, otherwise it will be for each item itself.

get_root(tree_id)

Returns the root node of tree with the given id.

insert_node(node, target, position='last-child', save=False)

Sets up the tree state for node (which has not yet been inserted into in the database) so it will be positioned relative to a given target node as specified by position (when appropriate) when it is inserted, with any neccessary space already having been made for it.

A target of None indicates that node should be the last root node.

If save is True, node's save() method will be called before it is returned.

move_node(node, target, position='last-child')

Moves node based on target, relative to position when appropriate.

A target of None indicates that node should be removed from its current position and turned into a root node. If node is a root node in this case, no action will be taken.

The given node will be modified to reflect its new tree state in the database.

For more details, see the move_to documentation above.

root_nodes()

Creates a QuerySet containing root nodes.

Example usage

In the following examples, we have Category and Question models. Question has a category field which is a TreeForeignKey to Category.

Retrieving a list of root Categories which have a question_count attribute containing the number of Questions associated with each root and all of its descendants:

roots = Category.tree.add_related_count(Category.tree.root_nodes(), Question,
                                        'category', 'question_counts',
                                        cumulative=True)

Retrieving a list of child Categories which have a question_count attribute containing the number of Questions associated with each of them:

node = Category.objects.get(name='Some Category')
children = Category.tree.add_related_count(node.get_children(), Question,
                                           'category', 'question_counts')