INTERNAL:
Specify that only a subset of the class' attributes be fetched in this query.
This allows for the query to be optimized through selecting less data.
Partial objects will be returned from the query, where the unspecified attributes will be left in their default value.
The primary key, and optimistic locking version (version or timestamp) if defined, will always be fetched.
The partial object is cached and can be modified.
An access through getter to an un-fetched attribute will trigger another SELECT to load the whole object.
Note: Modifying un-fetched attribute is not supported and exception would thrown.
getFetchGroupAttributeExpressions().add(attributeExpression);