MDM Search Expansion
Once you have MDM enabled, and you have many linked resources, it can be useful to search across all linked resources. Let's say you have the following MDM links in your database:
Patient/1 --> Patient/3
Patient/2 --> Patient/3
This indicates that both Patient/1 and Patient/2 are MDM-matched to the same golden resource (Patient/3). What if you want to get all observations from Patient/1, but also include any observations from all of their linked resources. You could do this by first querying the $mdm-query-links endpoint, and then making a subsequent call like the following
GET http://example.com:8000/Observation?subject=Patient/1,Patient/2,Patient/3
But HAPI-FHIR allows a shorthand for this, by means of a Search Parameter qualifier, as follows:
GET http://example.com:8000/Observation?subject:mdm=Patient/1
This :mdm parameter qualifier instructs an interceptor in HAPI fhir to expand the set of resources included in the search by their MDM-matched resources. The two above HTTP requests will return the same result.
On top of needing to instantiate an MDM module, you must enable this feature in the DaoConfig bean, using the Allow MDM Expansion property.