Summary

Enable class_derivations to be specified as a list, allowing multiple instances of the same target class to be created. This pattern is needed at two levels:

1. Top-level (TransformationSpecification): Multiple source tables → same target class
2. Slot-level (SlotDerivation): Single source row → multiple nested objects in a multivalued slot

Use Cases

Use Case 1: Multiple source tables to same target class

Different source tables from a study need to map to the same target class (e.g., condition data from multiple visits all becoming Condition instances).

class_derivations:
  - Condition:
      populated_from: pht004031  # Visit 1 conditions
      slot_derivations: ...
  - Condition:
      populated_from: pht004047  # Visit 2 conditions
      slot_derivations: ...

Currently achieved via external wrapper iteration in dm-bip, but should be natively supported.

Use Case 2: Nested object construction (MeasurementObservationSet)

A single source row needs to produce multiple nested objects. For example, one blood pressure row creates a MeasurementObservationSet containing two MeasurementObservation objects (systolic and diastolic).

class_derivations:
  MeasurementObservationSet:
    populated_from: pht001450
    slot_derivations:
      associated_participant:
        populated_from: phv00098771
      observations:
        class_derivations:
          - MeasurementObservation:
              slot_derivations:
                observation_type:
                  expr: "'OMOP:4154790'"  # Diastolic
                value_quantity:
                  class_derivations:
                    - Quantity:
                        slot_derivations:
                          value_decimal:
                            populated_from: phv00099392
                          unit:
                            expr: "'mm[Hg]'"
          - MeasurementObservation:
              slot_derivations:
                observation_type:
                  expr: "'OMOP:4152194'"  # Systolic
                value_quantity:
                  class_derivations:
                    - Quantity:
                        slot_derivations:
                          value_decimal:
                            populated_from: phv00099391
                          unit:
                            expr: "'mm[Hg]'"

Current Limitations

• YAML duplicate keys don't work (can't have two MeasurementObservation: keys)
• object_derivations was created as a workaround but adds unnecessary complexity
• External iteration wrappers are required for multi-source patterns

Implementation Notes

• Schema change: class_derivations should accept both dict and list forms
• Transformer should iterate over list and create instances for each entry
• Same pattern works at both TransformationSpecification and SlotDerivation levels

Related Issues

• Fix Measurement Observation Sets dm-bip#174 - Fix Measurement Observation Sets
• Implement data mapping from multiple classes dm-bip#222 - Implement data mapping from multiple classes
• Supersedes discussion in Revisit object_derivations #106