A flattened attribute is a special kind of attribute that
you effectively add from one entity to another by traversing a relationship.
When you form a to-one relationship between two entities (such as
Person and PersonPhoto), you can add attributes from the destination table
to the source table. For example, you can add a personPhoto attribute
to the Person entity. This is called “flattening” an attribute
and is equivalent to creating a joined column—it allows you to
create objects that extend across tables.