[Data-modeling] graph, nodes, relationships - a few questions

[Shawn Simister]

Roland Bouman wrote:
> Hi All!
>
> this is the first time I'm posting on the data-modeling list - i hope
> this is the right place.
>
> I am trying to understand MQL so I'm reading the MQL Reference Guide.
> I have a few questions about a few things I read, and I was hoping to
> get some help here.
> In particular, I am a tad confused about some of the things I read in
> http://mql.freebaseapps.com/ch02.html
>
> In "2.1. Nodes and Relationships"
> (http://mql.freebaseapps.com/ch02.html#id2942817) it reads:
>
> "...the Metaweb graph is a set of nodes and a set of links or
> relationships between those nodes.
> ....however, the nodes in the graph hold no information themselves.
> ....All the interesting data in the database is stored in the form of
> relationships between nodes (or between nodes and primitive values).
> ...Graphs can be represented visually using circles to represent nodes
> and arrows between the circles to represent relationships.
> "
>
> So far, so good. As I continue reading "2.2. Properties"
> (http://mql.freebaseapps.com/ch02.html#id2943383), I am confused by
> this:
>
> "...You may have wondered about the fact that these property
> identifiers look so much like node identifiers.
> ....this means, of course, [..] that properties are themselves nodes
> in the Metaweb graph.
> ....Since properties are nodes, they can appear in the From column of
> a table of tuples, and can have relationships themselves."
>
> Now, from what I understand, these "properties" are actually
> relationships (it's just represented as property in the tabular
> representation of the graph).
> But to me this sounds like it contradicts 2.1:
>
> One the one hand, nodes hold no information themselves, rather the
> relationships between nodes hold the interesting information, yet at
> the same time, relationships are themselves actually nodes.
> I also have trouble how this would be visualized "using circles to
> represent nodes and arrows between the circles to represent
> relationships".
> I mean, I can see how it is possible to still visualize a graph if you
> elect beforehand which nodes to draw as circles, and which ones as
> connecting lines,
> but my gut feeling says you cannot visually represent the same node
> both as a cirle and as a connecting line without duplication.
>
> Just to be clear - I am not interested in actually drawing graphs  - I
> am just looking for a way to understand this node/relationship
> duality.
>
> I'd greatly appreciate any pointers / explanations.
>
> kind regards,
>
>   
Check out the Thinkbase <http://thinkbase.cs.auckland.ac.nz/> app that 
uses Freebase data to draw the sort of graph diagrams that are talked 
about in chapter 2.

For example, see the graph for the movie Transformers 
<http://thinkbase.cs.auckland.ac.nz/index.html?q=/en/transformers> which 
was directed by Michael Bay. You can see that both the Transformers node 
and the Michael Bay node are connected with a line and if you mouse over 
that line it will say "Directed By".

As you've correctly read, the Directed By property 
<http://thinkbase.cs.auckland.ac.nz/index.html?q=/film/film/directed_by> 
itself is also stored in the graph and can be represented as it's own 
graph in Thinkbase.

To show both of these graphs in the same diagram would certainly be 
confusing because you would need to show a lot more detail than what  is 
currently being shown. For example, in the Transformers diagram, each 
line is actually what Freebase calls a Link 
<http://thinkbase.cs.auckland.ac.nz/index.html?q=/type/link> which is a 
node unto itself. To accurately show the underlying structure of the 
Freebase graph, each link should really be drawn as its own node with 
lines connectimg to the source, target and and the master property 
nodes. However, in most cases it makes more sense to abstract this 
detail away from the user to give them a simpler view of the graph that 
is closer the the object-oriented models that most people are used to 
working with.

Don't worry if this makes your head hurt. It should. It's pretty geeky 
stuff that probably took some really smart people a long time to design.

Shawn

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