19 August 2011

The Object Graph

In the ORM world, e.g. Hibernate, the term Object Graph is used quite a bit and it is a powerful notion, however, I suspect that Object Graph often gets used without much thought to some of the underlying concepts.  Generally the term refers to an Object, usually a Domain Object which often contains other Domain Objects and/or Collections of other Domain Objects via composition.  With an ORM like Hibernate very complex Object Graphs can be read, altered and persisted with very little work, it is a effective concept that can make building applications both easy and elegant especially if you have the proficiency and forethought to design your mappings properly.

In order to talk more intelligently about some of the underlying concepts regarding the Object Graph it is helpful to get a few fundamentals of Graph Theory out of the way, if you know them feel free to skip ahead.  Actually this is a good opportunity to put them in a context that may be more intuitive for many developers.  Graph Theory is an abstraction, a framework if you will, built on top of Set Theory, however, we don’t need to get bogged down in a Set based definition here, I do encourage you to pursue this on your own as it is extremely seminal and becoming increasingly essential.  We just need to get four fairly simple graph concepts out of the way which can be explained easily and visually, they are: The difference between a simple graph and a multi graph, the difference between a cyclic graph and an acyclic graph, the definition of a directed graph, and the definition of a tree.   Let’s start with the graphs:

As you can see from the above diagram we have several different types of graphs, now it should be noted that these attributes can overlap, the Undirected Graph contains cycles, so it is cyclic as does the Directed Graph, the Multi-Graph1, by definition contains cycles so it is cyclic this one is also undirected.  The Acyclic Graph is undirected. So these attributes can combine, except for an acyclic multi-graph which by definition can’t exist, I think.

In order to limit complexity for our discussion we are going to limit our object graph to the case of a directed acyclic graph, which is also known as a DAG.  More specifically we will be considering a specific case of this known as a directed rooted tree, where edges have a direction away from the root. The following example shows a "rooted" DAG aka a tree:

As you can see the above tree is also a domain object graph it is also it is a directed graph where the direction indicates containment via composition, which can be thought of as a "has-a" relation.  Now in some cases using ORM’s like hibernate one might include a mapping from Address back to Person so the ORM knows how to manage the relation and this is a cycle but it is an artifact created by the ORM since all accesses to this object graph by the application are from the root (Person node).  Now that is not to say that there might not be other structural reasons for such a cycle, perhaps an application needs to hold a list of addresses, but needs to get some of the Person attributes, then in this use case the Address node becomes the "relative" root from which the application access occurs so the object graph would then have two roots and need a cycle to maintain that duality.

So in many cases in regards to the ORM entity model the object graph can be thought of as a tree, a rooted directed acyclic graph (DAG).  There is one important divergence from traditional graph theory which is the presence of collections in graphs, these form a special node that can have combinatorial enumerative properties, in the case of the various types of collections also it can have certain mapping properties in the case of a Map object. An example of a modified person object that might hold a collection of addresses is:

There are a number of ways to expand on these ideas further but I think I will leave those for future posts.

1 This is the graph that Euler came up with for famous the Seven Bridges of Königsberg problem. The bridges are shown in the top image.