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For this the Patient demographic search using multiple Ids is designed to retrieve all the related Patient information in a single API call. In the BFF, the view-model is then constructed based on both datasets to support the User Interface.
Use Include and reverse include when
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searching multiple levels of resources
When creating a lists spanning both parent and child resources many use cases are supported using the include and reverse include functionality specified by FHIR:
http://hl7.org/fhir/STU3/search.html#revinclude
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