When using REST API composite JSON, how to refer with external id

  • an existing record
  • to existing Master-Detail parent from a newly created child

We have the following objects:

  • Organization__c:
    • Has Organization__c that is an External ID
      • ID comes from another system
      • A bit confusing since the field name is the same
    • (A bunch of other fields)
  • Foobar__c:
    • Has Organization__c field as Master-Detail(Organization)
    • (A bunch of other fields)

We're trying to create Foobar objects from another system that knows only external id for Organization.

We're trying to POST to services/data/v42.0/composite/tree/Foobar__c with "Organization__r": {"Organization__c": "123456"} for organization reference but get the following error: {"hasErrors":true,"results":[{"referenceId":"ref-1","errors":[{"statusCode":"INVALID_FIELD","message":"Cannot reference a foreign key field Organization__r.","fields":[]}]}]}

I tried also with "Organization__c": {"Organization__c": "123456"} but got: [{"message":"Cannot deserialize instance of reference from START_OBJECT value { or request may be missing a required field at [line:1, column:398]","errorCode":"JSON_PARSER_ERROR"}] My hypothesis here is that by using Organization__c I'm trying to create a new Organization record and that fails because there are fields missing.

UPDATE/ADDITION: Approach with Organization__r": {"Organization__c": "123456"} seems to work both


1 Answer 1


I think you only have two options here. The first is a "chatty" solution where your first step is to query Organization__c for the record ID.


Given that record Id, you should be able to then POST your foobar__c records directly now that you have both the SF record Id and the External Id.

It's two API calls, but should work.

As an alternative, you can write a Custom Apex REST endpoint that effectively does the same lookup from Organization__c.Organization__c to SF record Id.

I don't think that there is any other way to get around looking up the SF Record Id of the Master.

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