Error when I populate lookup with external id using .put method

Question

You can populate lookup fields using an object with an external id field populated like so:

Custom_Object__c o = new Custom_Object__c(Account__r=new Account(External_ID__c='123'));
insert o; 

However, when I want to dynamically assign values using the .put method instead of the property, I receive an error:

Custom_Object__c co = new Custom_Object__c();
SObject new_obj = co.newSObject();
new_obj.put('Account__r', new Account(External_ID__c='123'));

From the above code I receive Invalid field Account__r for Custom_Object__c

Answer 1

You are so close, this is not because you are using an External Id, it is because you are not using the right SObject method.

I have been caught out by this many times as well.

The correct syntax is:

new_obj.putSObject('Account__r', new Account(External_ID__c='123'));