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A polymorphic association table is a database design pattern that allows a single table to associate with multiple other tables, where the related tables can be of different types.

Scenario: Multiple entities in the domain model have attachments. There is a requirement to have a single attachment table to store attachments of all entity types. Another requirement is to store the attachments in a network folder. This would be recorded in the attachment table with the file path.

I created an entity called Attachment that corresponds to the polymorphic association table.

public class Attachment
{
    public int Id { get; set; }
    public string FilePath { get; set; }
    public string RelatedEntityName { get; set; }
    public int RelatedEntityId { get; set; }
    public bool IsDeleted { get; set; }
}

There is an entity called CalendarEvent that has attachments. RelatedEntityName would be "CalendarEvent" for attachments related to CalendarEvent. By the way, I am not sure if it is correct to have FilePath inside the domain model. Anyway, I need a way to persist attachments in the data store.

The application has 4 layers.

• Domain
• Application
• Infrastructure
• Presentation

What is the proper way to implement saving attachments for CalendarEvent as per Domain-Driven Design? My current implementation is as follows.

I have a domain service called CalendarEventAttachmentService in the Domain layer, that accepts a DTO called AttachmentDto, which is also in the Domain layer. AttachmentDto has two properties; FileContent and FileName. CalendarEventAttachmentService has a method that looks like this:

public List<Attachment> CreateAttachments(CalendarEvent calendarEvent, List<AttachmentDto> attachmentDtos, string folderPath)
{ 
    return attachmentDtos.Select(attachment => new Attachment 
    { 
        FilePath = folderPath + attachment.FileName, 
        RelatedEntityName = "CalendarEvent", 
        RelatedEntityId = calendarEvent.Id, 
    }); 
}

In the Application layer, the CreateCalendarEvent method in the CalendarService calls the CreateAttachments method mentioned above, and then adds the returned list of Attachment to the AttachmentRepository and saves changes.

This brings me to the topic of aggregate roots. Here, I feel like CalendarService is an aggregate root but Attachment is not. I read that only aggregate roots are supposed to have a repository. However, I could not find a way to save attachments to the database without AttachmentRepository.