Chaining
Chained parameters allow filtering by the parameter of a resource linked via a reference.
Example:
GET /fhir/Encounter?subject:Patient.gender=male
Syntax
Forward chain element
An element of a forward chain is represented as
<reference parameter name>:<target resource type>
Any chain must finish with a terminal. A terminal is just a search parameter name.
Example:
GET /fhir/Encounter?subject:Patient.gender=male
GET /fhir/Encounter?subject:Patient.general-practitioner:Practitioner.family=Smith
Reverse chain element
An element of a reverse chain is represented as
_has:<source resource type>:<search parameter>
Any chain must finish with a terminal. A terminal is just a search parameter name.
Example:
GET /fhir/Observation?_has:Encounter:subject
Chain
A chain is composed of an arbitrary number of chain elements and finished with a chain terminal.
There are some rules to join chain elements:
Forward chain elements are joined using the
.
symbol;Reverse chain elements are joined using the
:
symbol;Different types of chain elements are joined using the
.
symbol.
If the last chain element is a reverse element, then a terminal is joined using the :
symbol, otherwise the .
symbol.
Examples:
GET /fhir/Encounter?subject:Patient.name
GET /fhir/Observation?_has:Encounter:subject:code
GET /fhir/Encounter?subject:Patient.organization:Organization.name
GET /fhir/Patient?_has:Organization:_has:Observation:subject:code
GET /fhir/Patient?patient:Patient._has:Group:member:_id=group1
GET /fhir/Encounter?_has:Encounter:subject.practitioner:Practitioner._id=prac1
Semantics
You can interpret chain elements as a semijoin. The difference between forward and reverse chains is where a reference is located: in a forward chain, a reference is located on the left side of a semijoin, and in a reverse chain, a reference is located on the right side of a semijoin.
Example 1:
GET /fhir/Organization?_has:Patient:organization:_has:Observation:subject:code=...
You can read it as (pseudocode)
SELECT *
FROM Organizaton AS organizaton
SEMI JOIN Patient AS patient
ON patient.organization REFERS TO patient
SEMI JOIN Observation AS observation
ON observation.subject REFERS TO patient
WHERE observation.code = ...
Semantically, this means: find all organizations that have patients who have an observation with the given code.
Example 2:
GET /fhir/Observation?subject:Patient.organization:Organization.name=...
You can read it as (pseudocode)
SELECT *
FROM Observation AS observation
SEMI JOIN Patient AS patient
ON observation.subject REFERS TO patient
SEMI JOIN Organization AS organization
ON patient.organization REFERS TO organization
WHERE organization.name = ...
This means: find all observations that belong to some patient managed by some organization with the given name.
Example 3:
GET /fhir/Patient?_has:Encounter:subject.practitioner:Practitioner._id=...
You can read it as (pseudocode):
SELECT *
FROM Patient AS patient
SEMI JOIN Encounter as encounter
ON encounter.subject REFERS TO patient
SEMI JOIN Practitioner AS practitioner
ON encounter.practitioner REFERS TO practitioner
WHERE practitioner.id = ...
This means "find all patients who visited the given practitioner".
See also:
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