Search by multiple resource types

[fredhersch]

Is your feature request related to a problem? Please describe.
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Describe the solution you'd like

Search with joins of different types of resources within a single search:

• result[] = Get all Patients Where Observation.Code = ""
• See https://www.hl7.org/fhir/observation.html

Other examples:

As a CHW, show me all of the Patients currently enrolled in the ANC program

• ANC-Code = Lookup ValueSet (see Enable ValueSet expansion in the Search API so developers can search by ValueSet rather than disjunction of codes #609)
• Search for Patients Where EpisodeOfCare.type = ANC-Code.value AND Status.equals("Active") SORT BY DESC

Describe alternatives you've considered
Implement two-step search of:

1. Get a collection of Patients
2. For each Patient, search by Resource Type for the specific item

[jingtang10]

Examples:

• patients with latest risk assessmenet e.g. high
• patients with condition diabetes