How to refer to an existing record in REST API composite JSON?

Question

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

• When inserting a single Foobar object with REST API (POST to /services/data/v42.0/sobjects/Foobar__c)
  • See e.g. Give lookup field External Id in JSON request
• With Bulk API
  • See e.g. https://developer.salesforce.com/docs/atlas.en-us.api_asynch.meta/api_asynch/datafiles_xml_rel_fields.htm under "Relationship Fields for Custom Objects"

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.

/services/data/v20.0/query/?q=SELECT+Id+from+Organization__c+where+Organization__c+=+'<yourExternalId>'

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.