Typically, a business object that contains a single-cardinality child business object has at least two attributes that represent the relationship. The type of one attribute is the same as the child's type. The other attribute is a simple attribute that contains the child's primary key as a foreign key in the parent. The parent has as many foreign-key attributes as the child has primary-key attributes.
Because the foreign keys that establish the relationship are stored in the parent, each parent can contain only one single-cardinality child of a given type.
Figure 2 illustrates a typical single-cardinality relationship. In the example, fk1 is the simple attribute that contains the child's primary key, and child[1] is the attribute that represents the child business object.