How to create an EmailMessage record with DML having HasAttachment to be 'true'?

Question

I'm creating EmailMessage records via DML

EmailMessage createWhatEmail = new EmailMessage(
     Subject                = 'test'
    ,Status                 = '3'
    ,ToAddress              = 'heim@elastify.eu'
    ,FromAddress            = UserInfo.getUserEmail()
    ,FromName               = UserInfo.getFirstName()+' - '+UserInfo.getLastName()
    ,HtmlBody               = 'a body'
    ,Incoming               = false
    ,MessageDate            = DateTime.now()
    ,RelatedToId            = taskWhatId
    // ,HasAttachment       = true // not possible: Read-Only!
);
insert createWhatEmail;

The attachments I'm assigning like this:

for(ContentVersion cv : attachmentCVs) {
    ContentDocumentLink cdl = new ContentDocumentLink(
         ContentDocumentId  = cv.ContentDocumentId
        ,LinkedEntityId     = createWhatEmail.Id
        ,ShareType          = 'V' 
    );
    attachmentCDLs.add( cdl );
}

But the result is not flawless:

The attachment is shown correctly under the Attachments related list. But the non-writable HasAttachment field is false. Hence the paper-clip indicating the attachment isn't show in the Activity Tab under LEX.

Does anyone know a way to make this right?

The result should look more like this:

Answer 1

I spent close to 4 hours on this and figured it out. The key thing is to populate Attachment.ParentId with EmailMessage Id.

Here is the sample code. Make sure that you have a VF page named "DummyPage" before running the below code:

EmailMessage emailMessage = new EmailMessage();
emailMessage.status = '3'; // email was sent
emailMessage.relatedToId = '001XXXXXXXXXXX'; 
emailMessage.fromAddress = 'billing@acme.com'; // from address
emailMessage.fromName = 'Billing Department'; // from name
emailMessage.toAddress = 'johnSmith@example.com';
emailMessage.Subject = 'Hello';

emailMessage.HtmlBody = 'Hello World';
insert emailMessage;

Pagereference dummyPage = Page.DummyPage;
Blob blobObj = dummyPage.getContentAsPDF();

Attachment obj = new Attachment();
obj.Body = blobObj;
obj.ParentId = emailMessage.Id;
obj.Name = 'Dummy.pdf';
obj.ContentType = 'application/pdf';
insert obj;

System.debug([SELECT Id,HasAttachment FROM EmailMessage WHERE Id =: emailMessage.Id]);