message:
  tableName: messages
  columns:
    id:
      type: integer(4)
      primary: true
      unsigned: true
      autoincrement: true
    sender_id:
      type: integer(4)
      unsigned: true
    receiver_type: 
      type integer(4)
      unsigned: true
    receiver_id:
      type: integer(4)
      unsigned: true
    subject:
      type: string(512)
    message:
      type: string(4096)
  relations:
    sender:
      class: user
      local: sender_id
      foreign: id

userMessage:
  inheritance:
    extends: message
    type: column_aggregation
    keyField: receiver_type
    keyValue: 1
  relations:
    receiver:
      class: user
      local: receiver_id
      foreign: id

groupMessage:
  inheritance:
    extends: message
    type: column_aggregation
    keyField: receiver_type
    keyValue: 2
  relations:
    receiver:
      class: group
      local: receiver_id
      foreign: id