I came upon this error when I am assigning the custom object to the master standard object.

Eg. accountId derived from a query

Then I get this error.

The fix was actually easy. I just rewrote it to assign the master id on instantiating the child object.

Hope somebody finds this useful.

5 responses to “Master-Detail: Field is not writable error on Apex”

  1. Abinaya says:

    This solution is very helpful and easy also. Thanks 🙂

  2. lopau says:

    Glad you found it helpful.

  3. uma says:

    Thank you so much for this blog.Glad I found it

  4. sahil goyal says:

    What if we want to update the Account__c field ??

  5. lopau says:

    The example is for an insert DML. For update operation as the child record already exists, updating the record relationship should work fine as is.

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