Question or Change Request: How to generate a leaf where only specific nodes can be added through the model

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Description

Hi,
i started to use django-polymorphic-tree since two days and it's exactly that what i need for my current project. So at first thank you really for developing it.

I work with django since 5 months so my experience is not progressed. Maybe you can help me get the result that i want.

I would like to descipe a model that has following nodes:

• Projects
• Folders
• Files

With fancytree or jstree I would like to enable the users to create new projects all (on top level). Inside a project the user should be able to create folders and inside a folder the user should be able to create files and so on.

First i created following test for this case:

class BaseTreeNode(PolymorphicMPTTModel):
    parent = PolymorphicTreeForeignKey('self', blank=True, null=True,
                                       related_name='children', verbose_name='parent')
    name = models.CharField(max_length=33)
    uuid = UUIDField(version=4, auto=True)

class Project(BaseTreeNode):
    pass

class Folder(BaseTreeNode):
    pass

class File(BaseTreeNode):
    pass

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Test in console

p1 = Project.objects.create(name='Project1')
f1 = Folder.objects.create(name='Folder1', parent=p1)
fi1 = File.objects.create(name='File1', parent=f1)

p2 = Project.objects.create(name='Project2')
f2 = Folder.objects.create(name='Folder2', parent=p2)
fi2 = File.objects.create(name='File2', parent=f2)

p3 = Project.objects.create(name='Project3', parent=fi2)

def walk_the_tree(nodes, level=0):

    for node in nodes:
        print ('\t' * level) + '- <' + node.__class__.__name__ + '>' + node.name
        walk_the_tree(node.get_children(), level+1)

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Result

- <Project>Project1
    - <Folder>Folder1
        - <File>File1
- <Project>Project2
    - <Folder>Folder2
        - <File>File2
            - <Project>Project3

In this example Project 3 is added to a file. Currently it's possible but i would like to get an answer from you what is the best solution do handle this case?

Thank you

[vdboor]

Thanks for your praise and using this app!

You can set can_have_children = False on the File model to block adding nodes there.

If you want to further restrict which node types which the parent PolymorphicTreeForeignKey can point to, I'd recommend using a validator (or custom form) which blocks adding nodes of undesired types.

Note that validation only runs when you're using a ModelForm, or when model.full_clean() is run manually. It doesn't run with the .save() or .create() methods.

Thanks for your answer. It's correct with the validation for the forms it's possible but the form is not the only method that could create nodes. For example i want to write an importer (filesystem, xml, ..) that is using the models.py and additionaly i want to write an rest api.

There are different data sources that should modifiy the models and i would like to write a solution for all cases. Currently i have overwriten the save method of my base class:

class BaseTreeNode(PolymorphicMPTTModel):
    parent = PolymorphicTreeForeignKey('self', blank=True, null=True,
                                       related_name='children', verbose_name='parent')
    name = models.CharField(max_length=33)
    uuid = UUIDField(version=4, auto=True)

    parent_types = []

    def save(self, *args, **kwargs):
        """
        Save operation is only allowed for supported parent types
        :raise AttributeError: Throw Exception if parent type is not supported
        """
        if (self.parent.__class__.__name__ in self.parent_types) or (self.parent is None):
            super(BaseTreeNode, self).save(*args, **kwargs)
        else:
            raise AttributeError('Parent type %s is not supported. Supported types are: %s' % (
                self.parent.__class__.__name__, ', '.join(self.parent_types)))

What do you think about this solution or are there any cases on that i haven't thought yet?

[vdboor]

Protecting the model layer is the best way indeed. You can use the validations on the fields, you'll have to call the full_clean() method manually at the model to start the validation. So it becomes:

obj = Project(name='Project3', parent=fi2)
obj.full_clean()
obj.save()

This is a Django design decision or historical artifact, and can't easily be changed anymore (there have been discussions on the mailing list to let .save() validate the model, but these are rejected - for good reasons).

Thank you for your hint. i will use the clean for better django integration (Forms, ) and call the full_clean method befor save.