Search with related resources

A client can add related resources to a search result using (rev)include FHIR parameters and with Aidbox parameter. In ORM frameworks, such feature is sometimes called an "associations eager loading". This technique can save extra roundtrips from the client to the server and potential N+1 problem.
Example
This example demonstrates how _include, _revinclude, and _with search parameters work. You may want to get encounters with patients (each encounter refers to patient):
Create Patient 
PUT /Patient
​
resourceType: Patient
id: pat-234
name:
  - family: Smith
Create Encounter
PUT /Encounter
​
resourceType: Encounter
id: enc-234
subject: 
  resourceType: Patient
  id: pat-234
class: {code: 'IMP', 
  system: 'http://terminology.hl7.org/CodeSystem/v3-ActCode', 
  display: 'inpatient encounter'}
status: finished
Search with _include
Request
Response
GET /Encounter?_include=Encounter:subject:Patient
type: searchset
resourceType: Bundle
total: 1
link:
  - relation: first
    url: /Encounter?_include=Encounter:subject:Patient&page=1
  - relation: self
    url: /Encounter?_include=Encounter:subject:Patient&page=1
entry:
  - resource:
      class:
        code: IMP
        system: http://terminology.hl7.org/CodeSystem/v3-ActCode
        display: inpatient encounter
      status: finished
      subject:
        id: >-
          pat-234
        resourceType: Patient
      id: >-
        enc-234
      resourceType: Encounter
    search:
      mode: match
    fullUrl: [base]/Encounter/enc-234
    link:
      - relation: self
        url: [base]/Encounter/enc-234
  - resource:
      name:
        - family: Smith
      id: >-
        pat-234
      resourceType: Patient
    search:
      mode: include
    fullUrl: [base]/Patient/pat-234
    link:
      - relation: self
        url: [base]/Patient/pat-234
query-sql:
  - 'SELECT "encounter".* FROM "encounter" LIMIT ? OFFSET ? '
  - 100
  - 0
include-queries:
  - - SELECT * FROM "patient" WHERE (id in (?)) LIMIT ?
    - pat-234
    - 5000
Search with _with
Request
Response
GET /Encounter?_with=subject{Patient}
type: searchset
resourceType: Bundle
total: 1
link:
  - relation: first
    url: /Encounter?_with=subject{Patient}&page=1
  - relation: self
    url: /Encounter?_with=subject{Patient}&page=1
entry:
  - resource:
      class:
        code: IMP
        system: http://terminology.hl7.org/CodeSystem/v3-ActCode
        display: inpatient encounter
      status: finished
      subject:
        id: >-
          pat-234
        resourceType: Patient
      id: >-
        enc-234
      resourceType: Encounter
    search:
      mode: match
    fullUrl: [base]/Encounter/enc-234
    link:
      - relation: self
        url: [base]/Encounter/enc-234
  - resource:
      name:
        - family: Smith
      id: >-
        pat-234
      resourceType: Patient
    search:
      mode: include
    fullUrl: [base]/Patient/pat-234
    link:
      - relation: self
        url: [base]/Patient/pat-234
query-sql:
  - 'SELECT "encounter".* FROM "encounter" LIMIT ? OFFSET ? '
  - 100
  - 0
include-queries:
  - - SELECT * FROM "patient" WHERE (id in (?)) LIMIT ?
    - pat-234
    - 5000
​
Or you can request patients and return all Encounter resources that refer to them (by a reverse reference):
GET /Patient?_revinclude=Encounter:subject:Patient
Aidbox can do the same in a compact way:
GET /Patient?_with=Encounter.subject
Distinguish between matched and related resources
An entry.search.mode field has a value match if the resource is in the search set because it matched the search criteria and has a value include if  another resource refers to it.

_explain

Search Resource

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Outline

Example

Distinguish between matched and related resources