Fixed migration when having unsupported parent/child product nesting levels (fixes #1542)

changelog/63_UNRELEASED_xxxx.xx.xx.md

@@ -1,2 +1,4 @@
 - Fixed that the upgrade failed when having "> 2 times duplicate" (means the same barcode was added more than 2 times) product barcodes
 - Fixed that when having any "Track date only" chore on the calendar, the iCal export was broken
+- Fixed that the upgrade failed when having unsupported parent/child product nesting levels
+  - More information on this: Only 1 level is currently supported; creating > 1 level nestings was _never_ possible via the UI/frontend, but not checked/enforced by the backend before `v3.0.0` - so it was potentially possible via the API (or any third party app/tool which utilizes it) to create such a nesting which then made this upgrade to fail

migrations/0130.sql

@@ -1,3 +1,33 @@
+/*
+
+Parent/child product relations are currently limited to 1 level.
+
+This was enforced by the UI/frontend since ever, but on the backend/database only
+since v3.0.0 (by the trigger enfore_product_nesting_level).
+
+So via the API (or any third party app/tool which utilizes it), it was potentially possible
+to create > 1 level nestings before v3.0.0.
+
+The ALTER TABLE statement below does technically an update on all product rows (due to the column default value),
+so > 1 level nestings would make this fail.
+
+=> So clean up those unsupported nesting levels here.
+
+*/
+
+-- Clears parent_product_id for any > 1 level nesting (which is currently unsupported)
+UPDATE products
+SET parent_product_id = NULL
+WHERE id IN (
+	SELECT
+		p_child.id
+	FROM products p_parent
+	JOIN products p_child
+		ON p_parent.parent_product_id = p_child.id
+	WHERE p_parent.parent_product_id IS NOT NULL
+	)
+	AND parent_product_id IS NOT NULL;
+
 ALTER TABLE products
 ADD default_print_stock_label INTEGER NOT NULL DEFAULT 0;