Taxonomy tag API should return correct data for deeper levels

[jesperhodge]

Screen.Recording.2026-03-17.at.4.23.58.PM.mov

The taxonomy tag API GET, if receiving a full_depth_threshold = 1000, should return taxonomy tags of 4 levels: 0,...,3 (they are zero-indexed).
When a subtag is created for a tag that's at level 2, then the taxonomy tag API should include the new tag in its returned data. Currently it does not.
Creating a subtag at level > 3 (go to a tag at level 3, create new subtag) should return a 4xx error because of the MAX_DEPTH_LEVEL=3 setting on the backend.

Technical Details

To fix the bug, line 538 needs another one of those Q statements for the great grandparent: https://github.com/openedx/openedx-core/blob/848f6633fa4d1c8ede8db90c22a6d4eab485acf9/src/openedx_tagging/models/base.py#L533

When the parent_tag_value is none, the effect otherwise will exclude great-grandchildren.

[bradenmacdonald]

The taxonomy tag API GET, if receiving a full_depth_threshold = 1000, should return taxonomy tags of 4 levels: 0,...,3 (they are zero-indexed).
When a subtag is created for a tag that's at level 2, then the taxonomy tag API should include the new tag in its returned data. Currently it does not.
Creating a subtag at level > 3 (go to a tag at level 3, create new subtag) should return a 4xx error because of the MAX_DEPTH_LEVEL=3 setting on the backend.

I think we need to clear up the documentation here. I don't think TAXONOMY_MAX_DEPTH=3 means "we allow taxonomies up to 4 levels deep". I think as actually implemented it means "bulk APIs only work for tags up to 3 levels deep (Root->Child->Grandchild), but we technically allow tags of unlimited depth". See openedx/openedx-core#509 for my attempt to clarify this.

[bradenmacdonald]

Actually I have a better fix in progress here: openedx/openedx-core#510