Attachment:
  columns:
    title:
      type:       string(255)
    description:
      type:       clob
    url:
      type:       string(255)
    type: 
      type:       enum
      values:     [image, video, audio, document, application, link, model, custom, other]
      default:    other
    object_id:
      type:         integer
    object_class:
      type:         string(255)
      
ImageAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: image      
    
VideoAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: video      
    
AudioAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: audio      
    
DocumentAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: document
      
ApplicationAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: application
      
ModelAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: model    
    
LinkAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: link    
      
CustomAttachment:
  inheritance:
    extends: Attachment
    type: column_aggregation
    keyField: type
    keyValue: custom