166 Personal Content Experience: Managing Digital Life in the Mobile Age reading direction. For example, the contains relationship, when read from left-to-right, indicates that the fi rst object is a container for the second one. Reading from the opposite direction, the fi rst object is now a part of or belongs to the second object. Another way to limit the increase of the relationships is closer to a usage guidance than a strict rule. Remember Eddie’s loot case? A rela- tionship called “part of Eddie’s loot” can be created and then added between every object forming Eddie’s loot. Every time a new item arrives, a new relationship between the new item and every old item in Eddie’s loot can be added. However, there is an easier and more effi cient method, using those virtual objects mentioned in section 5.3. A virtual object called “Eddie’s loot” can be created that would have a “contains” relationship between all objects that form the loot. When a new item is received, only one relationship is added (between the loot and the new object). The number of relationships in Eddie’s loot would be identical to the number of objects, since each item has exactly one relationship. Of course, items in Eddie’s loot may have other relationships with each other, or relationships with objects not part of the loot. 5.6.9 How to Handle Composite Objects Many, if not most, media fi les are actually composed of several separate objects. For example, a text document may contain embedded pic- tures. Sometimes the main object is basically empty (this is the case with all collections) but it still has its own metadata. Our approach to com- posite objects is to use relationships between objects. In our metadata management system all objects, regardless of whether they are inde- pendent media objects such as a jpeg image or a part of another object such as an image in a text fi le, are handled separately. They have their own metadata independently of other objects, and in the case of a composite object, there is a relationship that indicates whether the object is a part of another object or if it contains sub-objects. See the relationship ontology tables for details about different relationships. For example, a collection object has a contains relationship with all objects that belong to the said collection. Likewise, all objects that belong to the collection have an is part of relationship with the collec- tion (or, the contains relationship reversed). Naturally, one object may have several relationships with one or many other objects. Indeed, one of the most used relation in our framework is the is part of relation. However, this relation can be used beyond indicating embedded objects or hierarchal relationships between objects. It is also useful in limiting metadata to some fragments of the main object only.