Tag Trees

Tags can be nested using tag trees for detailed categorisation, with tags having parents, children and siblings.

Tags in tag trees denote parents using the forward slash character (/). For example, Animal/Mammal/Cat is a Cat with a parent of Mammal and grandparent of Animal.

To use a slash in a tag name, escape it with a second slash; for example the tag name Animal/Vegetable can be entered as Animal//Vegetable.

A custom tag tree model must be a subclass of tagulous.models.TagTreeModel instead of the normal tagulous.models.TagModel; for automatically-generated tag models, this is managed by setting the tree field option to True.

Tag Tree Model Classes

tagulous.models.TagTreeModel

Because tree tag names are fully qualified (include all ancestors) and unique, there is no difference to normal tags in how they are set or compared.

A TagTreeModel subclasses tagulous.models.TagModel; it inherits all the normal fields and methods, and adds the following:

Note

Field values are computed and set automatically in the save() method - so don’t try to use them until the tag has been saved.

parent

A ForeignKey to the parent tag. Tagulous sets this automatically when saving, creating missing ancestors as needed.

children

The reverse relation manager for parent, eg mytag.children.all().

label

A CharField containing the name of the tag without its ancestors.

Example: a tag named Animal/Mammal/Cat has the label Cat

slug

A SlugField containing the slug for the tag label.

Example: a tag named Animal/Mammal/Cat has the slug cat

path

A TextField containing the path for this tag - this slug, plus all ancestor slugs, separated by the / character, suitable for use in URLs. Tagulous sets this automatically when saving.

Example: a tag named Animal/Mammal/Cat has the path animal/mammal/cat

level

An IntegerField containing the level of this tag in the tree (starting from 1).

merge_tags(tags, children=False)

Merge the specified tags into this tag.

tags can be a queryset, list of tags or tag names, or a tag string.

If children=False, only the specified tags will be merged; tagged items will be reassigned to this tag, but if there are child tags they will not be touched. If child tags do exist, although the merged tags’ counts will be 0, they will not be cleared.

If children=True, child tags will be merged into children of this tag, retaining structure; eg merging Pet into Animal will merge Pet/Mammal into Animal/Mammal, Pet/Mammal/Cat into Animal/Mammal/Cat etc. Tags will be created if they don’t exist.

get_ancestors()

Returns a queryset of all ancestors, ordered by level.

get_descendants()

Returns a queryset of all descendants, ordered by level.

get_siblings()

Returns a queryset of all siblings, ordered by name.

This includes the node itself; if you don’t want it in the results, exclude it afterwards, eg:

siblings = node.get_siblings().exclude(pk=node.pk)

tagulous.models.TagTreeModelManager

A TagTreeModelManager is the standard manager for a tagulous.models.TagTreeModel; it is a subclass of tagulous.models.TagModelManager so provides those methods, but its queries return a tagulous.models.TagTreeModelQuerySet instead.

tagulous.models.TagTreeModelQuerySet

This is returned by the tagulous.models.TagTreeModelManager; it is a subclass of tagulous.models.TagModelQuerySet so provides those methods, but also:

with_ancestors()

Returns a new queryset containing the nodes from the calling queryset, plus their ancestor nodes.

with_descendants()

Returns a new queryset containing the nodes from the calling queryset, plus their descendant nodes.

with_siblings()

Returns a new queryset containing the nodes from the calling queryset, plus theirm sibling nodes.

Converting from to tree tags from normal tags

When converting from a normal tag model to a tag tree model, you will need to add extra fields. One of those (path) is a unique field, which means extra steps are needed to build the migration.

These instructions will convert an existing TagModel to a TagTreeModel. Look through the code snippets and change the app and model names as required:

1. Create a data migration to escape the tag names.

   You can skip this step if you have been using slashes in normal tags and want them to be converted to nested tree nodes.

   Run manage.py makemigrations myapp --empty and add:

   def escape_tag_names(apps, schema_editor):
       model = apps.get_model('myapp', 'Tagulous_MyModel_Tags')
       for tag in model.objects.all():
           tag.name = tag.name.replace('/', '//')
           tag.save()
   operations = RunPython(escape_tag_names)
   

2. Create a schema migration to change the model fields. Because paths are not allowed to be null, you need to add the path field as a non-unique field, set some unique data on it (such as the object’s pk), and then change the field to add back the unique constraint.

   To do this reliably on all database types, see Migrations that add unique fields in the official Django documentation.

   If you are only working with databases which support transactions, you can use a tagulous helper to add the unique field:

   1. When you create the migration, Django will prompt you for a default value for the unique path field; answer with 'x' (do the same for the label field when asked).

      Change the new migration to use the Tagulous helper to add the path field.

   2. Add the unique field:

      import tagulous.models.migrations
      ...
      
      class Migration(migrations.Migration):
          # ... rest of Migration as generated
          operations = [
              ...
              # Leave other operations as they are, just replace AddField:
          ] + tagulous.models.migration.add_unique_field(
              model_name='_tagulous_mymodel_tags',
              name='path',
              field=models.TextField(unique=True),
              preserve_default=False,
              set_fn=lambda obj: setattr(obj, 'path', str(obj.pk)),
          ) + [
              ...
          ]
      

   Warning

   Although add_unique_column and add_unique_field do work with non-transactional databases, it is not without risk. See Database Migrations for more details.

3. We have changed the abstract base class of the tag model, but Django migrations have no native way to do this. You will need to use the Tagulous helper operation ChangeModelBases to do it manually, otherwise future data migrations will think it is a TagModel, not a TagTreeModel.

   Modify the migration from step 2; if you followed the official Django documentation and have several migrations, modify the last one. Add the ChangeModelBases to the end of your operations list, as the last operation:

   import tagulous.models.migrations
   
   class Migration(migrations.Migration):
       # ... rest of Migration as generated
       operations = [
           # ... rest of operations
           tagulous.models.migrations.ChangeModelBases(
               name='_tagulous_mymodel_tags',
               bases=(tagulous.models.models.BaseTagTreeModel, models.Model),
           )
       ]
   

4. Create another data migration to rebuild the tag model and set the paths:

   def rebuild_tag_model(apps, schema_editor):
       model = apps.get_model('myapp', 'Tagulous_MyModel_Tags')
       model.objects.rebuild()
   operations = RunPython(rebuild_tag_model)
   

   If you skipped step 1, this will also create and set parent tags as necessary.

5. Run the migrations

You can see a working migration using steps 2 and 3 in the Tagulous tests, for Django migrations.