Override or clear individual attributes of a copied slot/class

[amc-corey-cox]

`CopyDirective` operates on whole sub-elements (slots/classes/enums). There's currently no way to copy a slot and then modify one of its attributes — e.g. change a slot's range from string to a class and clear the now-stale `pattern`.

Use case from #67 (turbomam): take a source schema's class `C` with string-range slot `S` (with a regex `pattern`), and emit a target schema where `S`'s range is changed to a structured object/class instance with the `pattern` removed.

Likely mechanism: `SlotDerivation.target_definition` (already exists) should support clearing/overriding attributes on a copied slot, not just on a freshly-defined one. Needs investigation of current behavior.

Spun out of #67 / #244.

[github-actions]

Summary

This issue tracks a missing feature: the ability to copy a slot/class via CopyDirective and then patch specific attributes of the copy — e.g., change range from string to a class and simultaneously clear the now-stale pattern.

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Origin and lineage

Issue/PR	Status	Role
#67 (turbomam)	Open	Original request: copy class C's slot S, change its range, remove its pattern
#244 (PR, amc-corey-cox)	Open	Fixes exclude/exclude_all bugs in CopyDirective; explicitly defers this feature as out of scope
#245	Open	Sibling: define semantics and implement CopyDirective.add
#246 (this issue)	Open	Tracks the slot/class attribute override feature

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What currently exists

SlotDerivation.target_definition and ClassDerivation.target_definition are both declared in the schema model (transformer_model.yaml) and are implemented in SchemaMapper, but with inconsistent semantics:

Field	Location in schema_mapper.py	Behavior
ClassDerivation.target_definition	lines 227–232	Merge-only: only fills in attributes that are None or empty on the target class
SlotDerivation.target_definition	lines 348–351	Full-replace: calls setattr for every field, including ones not explicitly specified (resetting them to their SlotDefinition defaults)

Neither gives you the patch semantics the use case needs: copy the source slot in full, then override only the named fields and leave everything else intact.

Why the gap exists

_derive_slot (lines 331–351) already re-copies the source slot from the source schema and can apply target_definition on top. However, target_definition's full-replace implementation would also silently zero out every unspecified attribute (e.g., description, required, pattern, annotations) that you wanted to preserve from the source copy.

Additionally, _copy_class (the CopyDirective path) and slot_derivations iteration (line 216) are completely independent. A slot that arrives via CopyDirective doesn't participate in the _derive_slot post-processing step unless it also has an explicit SlotDerivation entry.

The design gap

What the use case needs is something like:

class_derivations:
  C:
    populated_from: C
    copy_directives:
      - copy_all: true
    slot_derivations:
      S:
        range: MyClass     # override range
        clear:             # explicitly null these fields
          - pattern

Or equivalently, an overrides map on SlotDerivation (analogous to ClassDerivation.overrides, which already exists at schema_mapper.py:234–238):

slot_derivations:
  S:
    overrides:
      range: MyClass
      pattern: null

Note that ClassDerivation already has an overrides dict that is applied as a post-processing patch (lines 234–238 in schema_mapper.py). SlotDerivation currently has no equivalent.

Possible implementation approaches

1. Add overrides to SlotDerivation — mirror the existing ClassDerivation.overrides pattern. Cleanest fit with existing design; low schema-change surface.
2. Add a clear list to SlotDerivation — explicit field names to null after copy. Pairs naturally with overrides or with target_definition.
3. Change SlotDerivation.target_definition to patch semantics — only override non-default values specified in the dict. Breaking change but consistent with ClassDerivation.target_definition's merge behavior. Worth considering alongside fixing the existing inconsistency.
4. Allow CopyDirective to trigger per-slot SlotDerivation post-processing — when copy_directives copies a slot that also has an entry in slot_derivations, apply the derivation as a post-copy patch rather than a full re-derive from source.

Workaround (today)

Don't use CopyDirective. Write an explicit SlotDerivation for each slot you want to copy, using populated_from: S to copy the source slot and range: MyClass to override the range. target_definition: {range: MyClass} would also clear pattern (since it's not in the dict and defaults to None), but it will also clear every other unspecified source attribute — so it's only safe for freshly-defined slots, not copy+patch.

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Spun out of #67 / #244. Sibling of #245.