What kind of change to apply
The original external record
Fields mapped to MJ entity field names
OptionalMatchedID of an existing MJ record if matched
Target MJ entity name
OptionalUnmappedSource keys the record returned that have NO active field map — the "extra" fields
the target table has no column for (yet). Computed at MapSingleRecord as
keys(ExternalRecord.Fields) − active map SourceFieldNames. Empty/undefined in the
common case (everything mapped). When non-empty, the engine parks it as JSON in the
__mj_integration_CustomOverflow system column so a post-sync RSU pass can promote
pervasive keys to real columns. Backend staging only — see CustomOverflow.
A record that has been mapped from external fields to MJ entity fields