Usage of “aliased” in SQLAlchemy ORM

Question

From the SQLAlchemy ORM Tutorial:

You can control the names using the label() construct for scalar attributes and aliased for class constructs:

Answer 1

aliased() or alias() are used whenever you need to use the SELECT ... FROM my_table my_table_alias ... construct in SQL, mostly when using the same table more than once in a query (self-joins, with or without extra tables). You also need to alias subqueries in certain cases.

There's an example in the documentation: http://www.sqlalchemy.org/docs/orm/query.html?highlight=aliased#sqlalchemy.orm.util.AliasedClass

Answer 2

As @jd. said,mostly when using the same table more than once in a query. Example:

    dict_code_type, dict_code_status = aliased(DictCode), aliased(DictCode)
    query = Device.query \
      .join(dict_code_type, dict_code_type.codeValue == Device.deviceType) \
      .join(dict_code_status, dict_code_status.codeValue == Device.status) \
      .with_entities(Device.id, Device.deviceName, Device.status,
                   Device.deviceID, Device.deviceUsername, Device.token,
                   dict_code_type.codeLabel.label('deviceTypeLabel'),
                   dict_code_status.codeLabel.label('statusLabel'),  Device.createAt, Device.authType) \
    .filter(and_(dict_code_type.code == 'deviceType', dict_code_status.code == 'status'))