Representing No Relation

While much of this book will use UUID values for primary keys, plenty of other Room examples will use int, particularly with autoGenerate set to true, to have SQLite generate the keys.

However, this does not work well if those keys will be used as foreign key values, in cases where there may be no value for the relation.

For example, Category uses String for its id (created from a UUID), and we represented a root category by means of having null for its parentId value. That works because String fields can be null.

If, however, we used int, we have no way of representing the no-relation scenario. You cannot assign null to an int field in Java.

Hence, if you want to support the no-relation scenario, your foreign key field needs to allow for null values. If you want to use auto-generated SQLite identifiers, use Integer, not int.