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', on_delete=models.CASCADE, 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.
Please note that if you are using multi-inheritance, MPTTModel should usually be the first class to be inherited from:
class Genre(MPTTModel,Foo,Bar):
name = models.CharField(max_length=50, unique=True)
Since MPTTModel inherits from models.Model
, this is very important when you have “diamond-style” multiple inheritance : you inherit from two Models that both inherit from the same base class (e.g. models.Model
) . In that case, If MPTTModel is not the first Model, you may get errors at Model validation, like AttributeError: 'NoneType' object has no attribute 'name'
.
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', on_delete=models.CASCADE, 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', on_delete=models.CASCADE, 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 of1
.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
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.
Warning
Please take extra caution in case you keep the references to the in-database objects when using this field. In cases of insertion or other updates, where
django-mptt
will need to re-order the tree you will most likely be left with dangling references. Please see order_insertion_by gotcha for details.
Registration of existing models
The preferred way to do model registration in django-mptt
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, on_delete=models.CASCADE, blank=True, null=True).contribute_to_class(Group, 'parent')
mptt.register(Group, order_insertion_by=['name'])
MPTTModel instance methods
Subclasses of MPTTModel have the following instance methods:
get_ancestors(ascending=False, include_self=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).
If include_self
is True
, the QuerySet
will also include the
model instance itself.
Raises a ValueError
if the instance isn’t saved already.
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).
Raises a ValueError
if the instance isn’t saved already.
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.
Raises a ValueError
if the instance isn’t saved already.
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_family()
Returns a QuerySet
containing the ancestors, the model itself
and the descendants, in tree order.
Raises a ValueError
if the instance isn’t saved already.
get_next_sibling()
Returns the model instance’s next sibling in the tree, or None
if it
doesn’t have a next sibling.
Raises a ValueError
if the instance isn’t saved already.
get_previous_sibling()
Returns the model instance’s previous sibling in the tree, or None
if it doesn’t have a previous sibling.
Raises a ValueError
if the instance isn’t saved already.
get_root()
Returns the root node of the model instance’s tree.
Raises a ValueError
if the instance isn’t saved already.
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.
Raises a ValueError
if the instance isn’t saved already.
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). If moved without any exceptions
raised then the signal node_moved
will be sent.
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 beforetarget
in the tree structure.'right'
The instance being moved should have
target
’s parent set as its new parent and should be placed directly aftertarget
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
, TreeOneToOneField
, TreeManyToManyField
Added in version 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.
There are also TreeOneToOneField
and TreeManyToManyField
if you need them.
These may come in useful on other models that relate to your tree model in some way.
Note
You can’t use a many-to-many as your ‘parent’ field. That’s because the mptt algorithm only handles trees, not arbitrary graphs. A tree where nodes can have multiple parents isn’t really a tree at all.
The TreeManager
custom manager
The default manager for an MPTTModel is a TreeManager
.
Any QuerySet
created with this manager will be ordered based on the
tree structure, with root nodes appearing in tree id order and their
descendants being ordered in a depth-first fashion.
Methods
The following manager methods are available:
disable_mptt_updates()
and delay_mptt_updates()
These two methods return context managers, and are both for doing efficient bulk updates of large trees. See the autogenerated docs for more information:
rebuild()
Rebuilds the mptt fields for the entire table. This can be handy:
if your tree gets corrupted somehow.
After large bulk operations, when you’ve used
disable_mptt_updates
It is recommended to rebuild the tree inside a transaction.atomic()
block
for safety and better performance.
Example usage in the admin
from mptt.admin import DraggableMPTTAdmin
from .models import Category, Product
class CategoryAdmin(DraggableMPTTAdmin):
mptt_indent_field = "name"
list_display = ('tree_actions', 'indented_title',
'related_products_count', 'related_products_cumulative_count')
list_display_links = ('indented_title',)
def get_queryset(self, request):
qs = super().get_queryset(request)
# Add cumulative product count
qs = Category.objects.add_related_count(
qs,
Product,
'category',
'products_cumulative_count',
cumulative=True)
# Add non cumulative product count
qs = Category.objects.add_related_count(qs,
Product,
'categories',
'products_count',
cumulative=False)
return qs
def related_products_count(self, instance):
return instance.products_count
related_products_count.short_description = 'Related products (for this specific category)'
def related_products_cumulative_count(self, instance):
return instance.products_cumulative_count
related_products_cumulative_count.short_description = 'Related products (in tree)'
root_node(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 necessary 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.objects.add_related_count(Category.objects.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.objects.add_related_count(node.get_children(), Question,
'category', 'question_counts')