ModelTree

ModelTree is a layer that sits atop of the Django ORM providing APIs for dynamically generating QuerySets at runtime. It manages figuring out all necessary joins between tables based on the relationships defined in each Model. The models below are all related in some way:

Install

Install using Pip or easy_install:

pip install modeltree

Setup

Add it to INSTALLED_APPS in your project settings:

INSTALLED_APPS = (
    ...
    'modeltree',
)

Define a default modeltree in your project settings:

MODELTREES = {
    'default': {
        'model': 'myapp.SomeModel',
    }
}

See how it looks by previewing the traversal tree:

python manage.py modeltree preview

The API

Let's start with a few example models:

from django.db import models

class Office(models.Model):
    location = models.CharField(max_length=50)


class Title(models.Model):
    name = models.CharField(max_length=50)
    salary = models.IntegerField()


class Employee(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    title = models.ForeignKey(Title)
    office = models.ForeignKey(Office)
    is_manager = models.BooleanField(default=False)


class Project(models.Model):
    name = models.CharField(max_length=50)
    employees = models.ManyToManyField(Employee)
    manager = models.OneToOneField(Employee, related_name='managed_projects')
    due_date = models.DateField()


class Meeting(models.Model):
    attendees = models.ManyToManyField(Employee)
    project = models.ForeignKey(Project, null=True)
    office = models.ForeignKey(Office)
    start_time = models.DateTimeField()
    end_time = models.DateTimeField()

In most cases the way relationships are traversed (the SQL joins that occurs) are predictable. Thus we can determine the shortest "path" between models.

>>> from modeltree.tree import trees
>>> from modeltree.utils import print_traversal_tree
>>> mt = trees.create(Office)
>>> mt
<ModelTree for Office>

>>> print_traversal_tree(mt)
Office
....Employee (via employee_set)
........Project (via project_set)
........Title (via title)
....Meeting (via meeting_set)

The hierarchy above shows the relationships relative to the Office model. We can easily query relative to the Office model.

Let's ask the question, "show me all the offices that have employees with the salary greater than 50,000". We need to use a special class here inspired by the Q class:

>>> from modeltree.utils import M
>>> m = M(mt, title__salary__gt=50000)
>>> queryset = Office.objects.filter(m)
>>> str(queryset.query)
'SELECT "tests_office"."id", "tests_office"."location" FROM "tests_office" INNER
JOIN "tests_employee" ON ("tests_office"."id" = "tests_employee"."office_id")
INNER JOIN "tests_title" ON ("tests_employee"."title_id" = "tests_title"."id")
WHERE "tests_title"."salary" > 50000 '

The lookup keyword argument title__salary__gt=50000 is a bit different than the normal lookups. This syntax is <app>__<model>__<field>__<operator>=<value>. The <app> and <operator> portions are optional. <app> is only required if there is an ambigiously named model, i.e. two apps have a model of the same name.

Since we are supplying the ModelTree instance with the lookup, it knows to build the QuerySet relative to the Office model.