The $diff operation can be used to generate a differential between two versions of a resource, or even two different resources of the same type.
Differentials generated by this operation are in FHIR Patch format.
In generated differentials, where a value has changed (i.e. a replace operation), an additional part value will be present on the given operation called previousValue. This part shows the value as it was in the from version of the resource.
When the $diff operation is invoked at the instance level (meaning it is invoked on a specific resource ID), it will compare two versions of the given resource.
fromVersion=[versionId]: (optional) If specified, compare using this version as the source. If not specified, the immediately previous version will be compared.includeMeta=true: (optional) If specified, changes to Resource.meta will be included in the diff. This element is omitted by default.To invoke:
GET http://fhir.example.com/baseR4/Patient/123/$diff
The server will produce a response resembling the following:
{
"resourceType": "Parameters",
"parameter": [ {
"name": "operation",
"part": [ {
"name": "type",
"valueCode": "replace"
}, {
"name": "path",
"valueString": "Patient.name.family"
}, {
"name": "previousValue",
"valueId": "Smyth"
}, {
"name": "value",
"valueId": "SmithB"
} ]
} ]
}
When the $diff operation is invoked at the server level (meaning it is invoked against the server base URL), it will compare two arbitrary resources of any type.
from=[reference]: Specifies the source of the comparison. The value must include a resource type and a resource ID, and can optionally include a version, e.g. Patient/123 or Patient/123/_history/2.to=[reference]: Specifies the target of the comparison. The value must include a resource type and a resource ID, and can optionally include a version, e.g. Patient/123 or Patient/123/_history/2.includeMeta=true: (optional) If specified, changes to Resource.meta will be included in the diff. This element is omitted by default.To invoke:
GET http://fhir.example.com/baseR4/$diff?from=Patient/1&to=Patient/2
The server will produce a response resembling the following:
{
"resourceType": "Parameters",
"parameter": [ {
"name": "operation",
"part": [ {
"name": "type",
"valueCode": "replace"
}, {
"name": "path",
"valueString": "Patient.id"
}, {
"name": "previousValue",
"valueId": "1"
}, {
"name": "value",
"valueId": "2"
} ]
} ]
}