Collection: swap nesting with Apply Rules

Tool

Use __APPLY_RULES__ to turn a list:list keyed as sample -> segment into a list:list keyed as segment -> sample.

When to reach for it

Use this when the upstream collection has the right leaf datasets but the wrong nesting axis for downstream map-over.

Do not use this for simple flattening; use collection-flatten-after-fanout when one axis should disappear. Do not use this to parse one identifier into multiple axes; use collection-split-identifier-via-rules.

This page is about pure axis swap. Use relabel-via-rules-and-find-replace when noisy identifiers must be cleaned during the reshape.

Parameters

The rule shape is intentionally small:

1. Add the outer identifier as a column.
2. Add the inner identifier as a column.
3. Map list identifiers in reversed order.

Conceptual Apply Rules shape:

tool_id: __APPLY_RULES__
tool_state:
  rules:
    - type: add_column_metadata
      value: identifier0
    - type: add_column_metadata
      value: identifier1
  mapping:
    list_identifiers: [1, 0]

Treat this as the logical shape, not a complete serialized workflow API blob.

Pitfalls

• Reverse the mapping, not the rules. The corpus shape adds identifier0, then identifier1, then maps [1, 0].
• Check that the input is actually list:list; identifier1 exists only when there is an inner list level.
• Keep pure regrouping separate from relabeling unless noisy identifiers force the heavier recipe.
• Downstream map-over behavior changes after the swap; tools now iterate by the former inner axis.

Footnote: the same workflow uses __DUPLICATE_FILE_TO_COLLECTION__ before one Apply Rules step as a broadcast setup. That broadcast is barely attested and should not be treated as the main pattern.

See also

• iwc-transformations-survey — Apply Rules Shape A and candidate boundary.
• galaxy-apply-rules-dsl — rule and mapping grammar.
• collection-split-identifier-via-rules — derive nesting axes from one identifier.
• collection-build-list-paired-with-apply-rules — promote a paired identifier into list:paired.