Merging and Unmerging Records: $merge and $unmerge

This page explains how to use merge and unmerge operations in MPI and how they work, with practical examples.

Overview

We use a hybrid merge strategy that combines elements of both the golden record and survivor record approaches:

• You have to select one of the existing records as the survivor.

• You can optionally edit its data using fields from the other records before completing the merge.

Currently, only manual merging is supported. However, the system is designed to support automatic merging in the future.

The unmerge operation allows reversing a previous merge by restoring the original source record and its relationships based on audit data, ensuring no permanent data loss if a merge was done by mistake.

This page provides key information about using $merge and $unmerge . For full API details, refer to our Swagger documentation.

If you need alternative merge and unmerge approaches to adjust MPI to your specific workflows and requirements, please contact us.

Merge Operation

$merge

The merge operation can be initiated either through the MPI user interface or by using the API.

To perform it via the API, send the $merge request, for example:

POST /fhir/Patient/$merge
Content-Type: application/json

{
  "targetPatient": {
    "reference": "Patient/0"
  },
  "sourcePatient": {
    "reference": "Patient/3"
  },
  "resultPatient": {
    "name": [
        {
            "given": ["Robert"],
            "family": "Alan"
        }
    ]
  }
}

Where:

• targetPatient – the patient record selected as the survivor. After the merge, this record remains active and contains the resulting data.

• sourcePatient – the patient record being merged into the survivor. After the merge, this record will be removed.

• resultPatient – optional. Provides updated data for the survivor (for example, if you want to correct a name, address, or any other field during the merge). If omitted, the survivor record remains unchanged, except for linked resources and identifiers that are merged automatically.

The response returns a merge-id used for audit and tracking:

{
  "merge-id": "b71dc614-7a1c-44b8-a727-027cbee3466a"
}

How Merging Works

When a merge request is processed, the system performs several steps to ensure data consistency and auditability:

1. Verifies the match weight exists

2. All related resources are found and updated

3. A merge record is created with status and match weight

4. All changes are logged for audit purposes

5. The target patient is updated with merged data

6. The source patient is deleted

7. The merge ID is returned to the client

The diagram below shows the full process flow:

Unmerge Operation

If a merge was performed incorrectly, it can be reversed using the unmerge operation.

This operation restores the original source patient and re‑links all related resources based on audit information collected during the merge.

If any linked resources were created after the merge, the operation also provides a way to manually reassign them to the appropriate patient.

$unmerge-preview/{id}

Before performing an unmerge, you should preview what will be changed using the Unmerge Review endpoint. This endpoint returns a list of encounters created after the merge, which may need to be manually reassigned during the unmerge process.

To get the list of after-merge linked resources via API, send the $unmerge-preview request, for example:

POST /fhir/Patient/$unmerge_preview/1
Content-Type: application/json

Where 1 is the merge ID returned by the $merge operation you want to undo.

Example response:

{
  "target-id": "patient-123",
  "source-id": "patient-456",
  "new-encounters": [
    {
      "id": "encounter-789",
      "resource": {
        "resourceType": "Encounter",
        "status": "finished",
        "class": {
          "system": "http://terminology.hl7.org/CodeSystem/v3-ActCode",
          "code": "AMB",
          "display": "ambulatory"
        }
      }
    }
  ]
}

Where:

• target-id – the patient ID of the survivor record after the merge.

• source-id – the patient ID of the merged (source) record that will be restored.

• new-encounters – list of encounters created after the merge that may need to be manually reassigned when performing an unmerge.

$unmerge

As well as the merge operation, the unmerge operation can be initiated either through the MPI user interface or by using the API.

To perform the unmerge via the API, send the $unmerge request, for example:

POST /fhir/Patient/$unmerge
Content-Type: application/json

{
  "merge-id": "1",
  "encounters-and-patient-ids": [
    {
      "patient-id": "patient-456",
      "encounter-ids": [
        "encounter-123"
      ]
    }
  ]
}

Where:

• merge-id – the unique identifier of the merge operation to be reversed.

• encounters-and-patient-ids – optional mapping of encounters created after the merge that need to be manually reassigned:

  • patient-id – the patient to whom encounters should be linked after the unmerge.

  • encounter-ids – list of encounters that should be re‑linked to this patient. This ensures that any encounters created post‑merge are correctly redirected when restoring the original patient records. You can get the list of post-merge encounters via $unmerge-preview/{id} request.

How Unmerging Works

Processing an unmerge request follows a specific workflow designed to safely restore previously merged data:

1. The system retrieves the original merge logs

2. Source and target patient information is extracted

3. Encounters are grouped by patient ID

4. Resources that need to be reverted are identified

5. Patient references are updated for selected encounters and other linked resources

6. Original resources are restored from merge logs

7. The merge record is removed

8. Patients are marked as non-duplicates

9. The operation is completed

The diagram below shows the full process flow: