Tuesday 17 March 2015

Hidden GraphGems: the Meta-graph

Over the past couple of days, I have had multiple occasions where I had to explain a graph model to an audience. Often times I would end up drawing stuff on paper/whiteboard (still my favourite, tbh), sometimes I would revert back Arrows, but more and more I am in a situation where I need to explain the model of an **existing Neo4j database**. So what then?

Well, that's when I pull out a old hidden gem, as explained and handed to me by Michael, of course. He explained it very well on this graphgist, but let me try to take you through it.

Starting from a standard query

Every Neo4j instance that has the Neo4j browser (so since version 2.0), has had the following query shipped with it under the "favourites": what is related, and how.

 // What is related, and how  
 MATCH (a)-[r]->(b)  
 WHERE labels(a) <> [] AND labels(b) <> []  
 RETURN DISTINCT head(labels(a)) AS This, type(r) as To, head(labels(b)) AS That  
 LIMIT 10  

Essentially what this does is
  • look at all the (a)-[r]->(b) patterns
  • ensure that the nodes have a label
  • and then take the labels of (a) and the labels of (b), and the type of [r]
that gives you a simple table that explains the model of the database:

But what if I want to do what I said before - and really show the model, graph-ically? Well, as explained in Michael's gist, you can tweak the query above to do that.

Tweaking the query to generate the META-graph

Here's what you do to the query:

 // generate the META-graph  
 MATCH (a)-[r]->(b)  
 WITH labels(a) AS a_labels,type(r) AS rel_type,labels(b) AS b_labels  
 UNWIND a_labels as l  
 UNWIND b_labels as l2  
 MERGE (a:Node:Meta {name:l})  
 MERGE (b:Node:Meta {name:l2})  
 MERGE (a)-[:OUTGOING]->(:Relationship:Meta {name:rel_type})-[:INCOMING]->(b)  
 RETURN distinct l as first_node, rel_type as connected_by, l2 as second_node  

Let's take you through that too:
  • we look at the same (a)-[r]->(b) pattern as above
  • we collect all the labels of (a), types of [r] and labels of (b)
  • we unwind these into l and l2
  • and then start looping through these using MERGE: we create a node (a) for every new label l, a node (b) for every new label l2, and leave them alone if they already exist
  • and then we also create a node for every relationship between a and b. Note that this may take a bit of getting used to in reading the model: relationships are expressed as nodes, and connected to labels with an "outgoing" and "incoming" relationship.
Overall this yields a very readable model diagram: all the META-nodes have a label "Meta", and that makes for easy visualisation. I my example below, the yellow nodes are "labels" and the blue nodes are "relationships". Very readable, I believe.

To me, this is standard equipment for dealing with existing graph models/databases.

Hope you like it, and that it is useful for you too. Massive kudos to Michael, of course - but that almost goes without saying.



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