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django-tree-queries
===================
:alt: CI Status
Query Django model trees using adjacency lists and recursive common
table expressions. Supports PostgreSQL, sqlite3 (3.8.3 or higher) and
MariaDB (10.2.2 or higher) and MySQL (8.0 or higher, if running without
ONLY_FULL_GROUP_BY).
Supports Django 3.2 or better, Python 3.8 or better. See the GitHub actions
build for more details.
Features and limitations
========================
- Supports only integer and UUID primary keys (for now).
- Allows specifying ordering among siblings.
- Uses the correct definition of depth, where root nodes have a depth of
zero.
- The parent foreign key must be named ``"parent"`` at the moment (but
why would you want to name it differently?)
- The fields added by the common table expression always are
tree_depth, tree_path and tree_ordering. The names cannot
be changed. tree_depth is an integer, tree_path an array of
primary keys and tree_ordering an array of values used for
ordering nodes within their siblings. Note that the contents of the
tree_path and tree_ordering are subject to change. You shouldn't rely
on their contents.
- Besides adding the fields mentioned above the package only adds queryset
methods for ordering siblings and filtering ancestors and descendants.
Other
features may be useful, but will not be added to the package just because
it's possible to do so.
- Little code, and relatively simple when compared to other tree
management solutions for Django. No redundant values so the only way
to end up with corrupt data is by introducing a loop in the tree
structure (making it a graph). The TreeNode abstract model class
has some protection against this.
- Supports only trees with max. 50 levels on MySQL/MariaDB, since those
databases do not support arrays and require us to provide a maximum
length for the tree_path and tree_ordering upfront.
- **Performance optimization**: The library automatically detects simple
cases
(single field ordering, no tree filters, no custom tree fields) and uses
an
optimized CTE that avoids creating a rank table, significantly improving
performance for basic tree queries.
Here's a blog post offering some additional insight (hopefully) into the
reasons for `django-tree-queries' existence
`_.
Usage
=====
- Install ``django-tree-queries`` using pip.
- Extend ``tree_queries.models.TreeNode`` or build your own queryset
and/or manager using ``tree_queries.query.TreeQuerySet``. The
TreeNode abstract model already contains a parent foreign key
for your convenience and also uses model validation to protect against
loops.
- Call the ``with_tree_fields()`` queryset method if you require the
additional fields respectively the CTE.
- Call the ``order_siblings_by("field_name")`` queryset method if you want
to
order tree siblings by a specific model field. Note that Django's
standard
``order_by()`` method isn't supported -- nodes are returned according to
the
`depth-first search algorithm
`__.
- Use ``tree_filter() and tree_exclude()`` for better performance when
working with large tables - these filter the base table before building
the tree structure.
- Use ``tree_fields()`` to aggregate ancestor field values into arrays.
- Create a manager using
``TreeQuerySet.as_manager(with_tree_fields=True)`` if you want to add
tree fields to queries by default.
- Until documentation is more complete I'll have to refer you to the
[test suite
]
for additional instructions and usage examples, or check the recipes
below.
Recipes
=======
Basic models
~~~~~~~~~~~~
The following two examples both extend the TreeNode which offers a few
agreeable utilities and a model validation method that prevents loops in
the
tree structure. The common table expression could be hardened against such
loops but this would involve a performance hit which we don't want -- this
is a
documented limitation (non-goal) of the library after all.
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