Create record via REST api with external id in master detail relationship

Question

We have a Java application integrated to Salesforce that can create a case using the REST API like this:

POST /services/data/v33.0/sobjects/Case

Body:

{"Subject":"Test Subject","RecordTypeId":"012200000003230AAA","Account":{"MerchantAccountNumber__c":"MAN004370053"}}

Instead of using the account id to attach it to the right account, we use the external id field MerchantAccountNumber__c. This works fine.

Now this application needs to create a custom object MerchantProcess__c that is also related to Account over a master-detail relationship called AccountName__c, but we cannot get the REST call to work:

POST /services/data/v33.0/sobjects/MerchantProcess__c/

{"AccountName__c":{"MerchantAccountNumber__c":"MAN4290050938"},"RecordTypeId":"0128E000000Cuki","DetailedInformation__c":"Details here"}

This returns:

[{"message":"Cannot deserialize instance of reference from START_OBJECT value { or request may be missing a required field at [line:1, column:2]","errorCode":"JSON_PARSER_ERROR"}]

How could I create a MerchantProcess__c record over the REST api and attach it to an account while using the external id MerchantAccountNumber__c on Account?

It would be possible to query the account id by the MerchantAccountNumber__c and then create the MerchantProcess__c in a second call, but I want to avoid this if possible to save on api calls and reduce complexity.

Answer 1

You need to use __r and not __c

{"AccountName__r":{"MerchantAccountNumber__c":"MAN4290050938"},"RecordTypeId":"0128E000000Cuki","DetailedInformation__c":"Details here"}

And I am assuming that you are using upsert here as the DML method but you have not shown your code so...