通过apex发送时，合并字段值不会显示在电子邮件模板中

Question

我有一个电子邮件模板，我想通过apex class发送。我收到一个电子邮件通知但合并字段的值没有显示。我尝试了很多解决方案，但没有得到结果。请问，有人可以帮我吗？

这是我的电子邮件模板Email template的快照

和顶点代码： -

global class sampleBatch implements Database.Batchable<sObject>{
global Database.QueryLocator start(Database.BatchableContext bc) {
    String query = 'Select id,Name from book__c;
    return Database.getQueryLocator(query);
}

global void execute(Database.BatchableContext bc, List<book__c> books){ 
    Map<Id, List<Case>> associatedCases = CaseHelperClass.getCase(new Map<Id,book__c>(books).keySet());
    Map<Id,book__c> bookMap= new Map<Id,book__c>(books);
    EmailTemplate emailTemplate = [SELECT Id,Subject,Description,htmlValue,DeveloperName,Body FROM EmailTemplate WHERE DeveloperName =: 'sameple_template'];
    if(associatedCases <> null){
        for(Id bookId : associatedCases.keySet()){
            String headerAndAssociatedCases = 'CaseNumber,Type,Status,Subject,Description,CreatedDate
';
            for(Case c : associatedCases.get(bookId)){
                headerAndAssociatedCases += c.CaseNumber+','+c.Type+','+c.Status+','+c.Subject+','+c.Description+','+c.CreatedDate+'
';
            }
            Messaging.EmailFileAttachment csvAttachment = new Messaging.EmailFileAttachment();
            blob csvBlob = Blob.valueOf(headerAndAssociatedCases);
            csvAttachment.setFileName('Case.csv');
            csvAttachment.setBody(csvBlob);
            Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
            list<string> toAddresses = new list<string>{name@test.com};
            email.setSubject(emailTemplate.Subject);
            email.setTemplateId(emailTemplate.Id);
            email.setPlainTextBody(emailTemplate.Body);
            email.setToAddresses(toAddresses);
            email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttachment});
            Messaging.SendEmailResult [] result = Messaging.sendEmail(new Messaging.SingleEmailMessage[] {email});
       }
    }
}

global void finish(Database.BatchableContext bc){
    system.debug('Apex Job Done');
}}

任何帮助都会非常值得一提。谢谢！