Virtual fields¶

Virtual fields are CDM-table fields that only exist inside the engine — they never reach the destination database. They are used as scratch space for normalisation, indexing, or holding intermediate values for joins.

Declaring a virtual field¶

Add a field: line to the Comments field of the CDM table in Rabbit-in-a-Hat:

field: visit_occurrence_source_value

The text after field: becomes the name of the virtual field. The field is then visible to every rule running on that CDM table — both field-to-field maps and CDM-field rules.

The example above adds a visit_occurrence_source_value field to visit_occurrence — a common requirement on CDM 5.4, where visit_occurrence lacks a *_source_value field for its primary key.

Writing to a virtual field from a field-to-field map¶

Use SaveToVirtualField on a transition that targets an otherwise-unrelated field; the rule redirects the value into the named virtual field:

SaveToVirtualField(:care_site_map)

This is necessary because Rabbit-in-a-Hat does not visualise virtual fields, so there is no transition arrow you could place a normal rule on.

Writing from rule chain output¶

Any rule chain's Output_Fields_List (the part after the trailing =>) can declare new fields. New fields declared this way become virtual fields and remain in scope for subsequent rules in the same chain:

:gender => Map([["M", 8507], ["F", 8532]]) => :result
:result => AssertNot(8532)
Use(:gender)

Here :result is a virtual field that lives only for the duration of the chain.