fix: make tag import parent_id handling correct and order-independent

src/openedx_tagging/import_export/actions.py

@@ -146,7 +146,7 @@ def _validate_parent(self, indexed_actions) -> ImportActionError | None:
                     action=self,
                     message=_(
                         "Unknown parent tag ({parent_id}). "
-                        "You need to add parent before the child in your file."
+                        "Add a row for the parent to your file."
                     ).format(parent_id=self.tag.parent_id),
                 )
         return None
@@ -343,12 +343,30 @@ def applies_for(cls, taxonomy: Taxonomy, tag: TagItem, indexed_actions=None) ->
         """
         This action applies whenever there is a change on the parent.
 
+        A row whose parent_id equals the current parent's external_id is "no
+        change" and skips `_validate_parent`. TagImportPlan validates the
+        invalid cases of such rows itself: a parent renamed away, and parent
+        cycles. The exception is a parent that this import deletes while a
+        rename reclaims its id: the row applies, to move the child under the
+        renamed tag.
+
         See ImportAction._resolve_update_target for when this doesn't apply
         regardless of the parent change (a rename_external_id row).
         """
         taxonomy_tag = cls._resolve_update_target(taxonomy, tag)
         if taxonomy_tag is None:
             return False
+        parent = taxonomy_tag.parent
+        # The parent is deleted by this import and a rename reclaims its id, so
+        # the child must move under the renamed tag instead of being promoted.
+        if (
+            parent is not None
+            and parent.external_id == tag.parent_id
+            and indexed_actions
+            and any(a.tag.id.casefold() == parent.external_id.casefold() for a in indexed_actions["delete"])
+            and any(a.tag.id.casefold() == parent.external_id.casefold() for a in indexed_actions["rename_external_id"])
+        ):
+            return True
         return (
             taxonomy_tag.parent is not None
             and taxonomy_tag.parent.external_id != tag.parent_id

src/openedx_tagging/import_export/exceptions.py

@@ -162,3 +162,52 @@ class InvalidCSVField(TagParserError):
     def __init__(self, tag: dict | None, field: str, row: int, **kargs):
         super().__init__(tag, **kargs)
         self.message = _("Invalid '{field}' field on the row {row}").format(field=field, row=row)
+
+
+class ParentIdRenamedAwayError(TagImportError):
+    """
+    Exception raised when a row's parent_id is its current parent's pre-import
+    id, and the same import renames that parent to a different id.
+
+    After the import that id names no tag or, after a swap of ids, a different
+    tag, so the message asks for the parent's new id instead. ``row`` is the
+    1-based position of the row among the file's data rows: row 1 is the first
+    row after a CSV header, or the first entry of a JSON file's ``tags``.
+    """
+
+    def __init__(self, row: int, parent_id: str, new_id: str, **kargs):
+        super().__init__(**kargs)
+        self.message = _(
+            "Row {row}'s parent_id ({parent_id}) names an id this file renames away. "
+            "Use '{new_id}' as the parent_id."
+        ).format(row=row, parent_id=parent_id, new_id=new_id)
+
+
+class ParentCycleError(TagImportError):
+    """
+    Exception raised when the parent_id of a row makes the tag its own
+    ancestor once the import is applied.
+
+    ``row`` is numbered like in ``ParentIdRenamedAwayError``.
+    """
+
+    def __init__(self, row: int, parent_id: str, **kargs):
+        super().__init__(**kargs)
+        self.message = _(
+            "Row {row}'s parent_id ({parent_id}) creates a parent cycle."
+        ).format(row=row, parent_id=parent_id)
+
+
+class InvalidParentHierarchyError(TagImportError):
+    """
+    Exception raised when applying the import would build a tag tree that is too
+    deep or otherwise invalid, even when the importer first moves every tag whose
+    parent changes to the root.
+    """
+
+    def __init__(self, max_levels: int, **kargs):
+        super().__init__(**kargs)
+        self.message = _(
+            "This import would leave tags deeper than {max_levels} levels, or build an invalid "
+            "parent hierarchy, so it cannot be applied."
+        ).format(max_levels=max_levels)