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.
This behaviour is also supported on the $everything operation, via a slightly different mechanism. If you call the operation with _mdm=true, then MDM expansion will occur on the base Patient instance. For example:
GET http://example.com:8000/Patient/1/$everything?_mdm=true
This will first lookup all Patients linked to Patient/1, and then perform an $everything including all resources for these patients.
On top of needing to instantiate an MDM module, you must enable this feature in the DaoConfig bean, using the Allow MDM Expansion property.