omop-alchemy currently enforces a shared ValueMixin rule requiring at least one of value_as_number or value_as_concept_id to be set

[gkennos]

This is too strict for OMOP CDM v5.4 and blocks valid rows in observation and measurement.

The current mixin adds:

• a DB check constraint: value_as_number IS NOT NULL OR value_as_concept_id IS NOT NULL
• an assignment-time validator raising ValueError("At least one of value_as_number or value_as_concept_id must be set")

This rule is inherited by:

• Observation
• Measurement
• Metadata

For Observation, OMOP allows the value to be represented via:

• value_as_concept_id
• value_as_number
• value_as_string

For Measurement, OMOP does not require a captured result value on every row. As a result, valid CDM rows can be rejected by the ORM even though they conform to OMOP.