About this document
This document describes the changes Django MPTT makes to your Django model classes and the custom tree manager it provides, so you can easily work with model instances as trees.
Contents
The mptt module contains a register function, which users can call to set a Django model up for MPTT. This function takes as its arguments the model class itself and additional options related to working with the model as a tree.
mptt.register and syncdb
As discussed further below, the mptt.register function will dynamically add new fields to your models if certain required tree fields are not present.
As such, unless you are manually setting up all required tree fields in your models, you must have the call to mptt.register in place before you use Django's syncdb management command to create tables. Otherwise, you will have to recreate tables so the appropriate tree columns are in place in the database.
The following argument is required:
All remaining arguments are optional, but you should take care to specify appropriate fields names where the default field names do not fit with your model class' definition:
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 = models.ForeignKey('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:
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.
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.
The name of a field which should define ordering when new tree nodes are being inserted or existing nodes are being reparented. Defaults to None.
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.
A mimimal example usage of mptt.register is given below, where the model being set up for MPTT is suitable for use with the default arguments which specify fields and the tree manager attribute:
from django.db import models
import mptt
class Genre(models.Model):
name = models.CharField(max_length=50, unique=True)
parent = models.ForeignKey('self', null=True, blank=True, related_name='children')
mptt.register(Genre, order_insertion_by='name')
The following instance methods will be added to your Django models when you set them up for MPTT:
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', commit=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 commit is True, the model instance's save() method will be called before the instance is returned.
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
Is 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 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.
A custom manager, TreeManager is also added to your Django models when you set them up for MPTT - the attribute this manager can be accessed through is specified by the tree_manager_attr argument to mptt.register.
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.
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.
A Django model class which has a relation to this manager's model.
The name of the field in rel_cls which holds the relation.
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.
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', commit=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 commit 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.
In the following examples, we have Category and Question models. Question has a category field which is a ForeignKey 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')