One of the primary purposes of this blog is to record what I’ve learned by tedious trial and error and/or spending time down in source code I shouldn’t have had to look at.
This particular topic has more than its share of discoveries.
Spring Neo4j claims that it’s intended to imitate, where possible, existing persistence systems approaches. Unfortunately, it has a long way to go on that.
First, let me mention that after descending through Maven’s equivalent of “DLL Hell”, I have been working with the following version sets:
- Spring Framework version 4.0.6
- Spring Neo4j 3.3.0.Release
- Neo4j version 2.2.0
- Logging courtesy of slf4j version 1.7.6 and log4j2 version 2.2, which has renamed the log4j config files since log4j v1 and added a few new config options (just for info).
As usual, just finding what was compatible with what was an adventure. I have a nasty, if unprovable suspicion that some of the pain could have been reduced if certain functions and classes had been marked “deprecated” instead of being removed or relocated.
Some of what I’ve learned will break shortly. Spring Neo4j 4 will actually lose a number of functions and annotations that exist in Spring Neo4j 3 (they were probably broken anyway). And some of the fixes I’ve come up with may actually be exploiting bugs rather than clean fixes. It’s the best I could do.
My use case seemed simple. I have 2 entity types: A and B, and a relationship, we’ll call “wants” which has a property named “priority”. I want to set up a network of many-to-many A-wants-B relationships with associated priorities and I want to be able to select a given B and get back an ordered list of A-wants in priority order with the priorities visible.
So using Spring Data Neo4j (SDN, for short), that gives me 2 NodeEntity classes (A and B) and a RelationshipEntity (“Wants”). I’m using GraphRepository extensions to manage persistence for A and B, which gives rise to the first question:
Q: What’s the best way to handle persistence of RelationshipEntities?
There’s a RelationshipGraphRepository, but unlike GraphRepository, it’s a class, not an interface and there is absolutely zero documentation I can find on how to use it or even if I should be explicitly using it. I therefore used ordinary GraphNode for the relationshipEntity. It seems to work. More or less.
And the first observation:
When they say “@Fetch”, they mean “@Fetch”
In JPA, a recommended way of persisting an object is in the form “a = repository.save(a);” This is because JPA may construct new new “a” using the original “a” as a basis, but carrying (visibly or not) extra data and/or meta-data provided by the “save” operation.
This is also the (explicitly) recommended practice for SDN. But there’s a big “gotcha”.
When you persist with JPA, the returned object will contain AT LEAST as much data as the original copy did (it may even be the original copy). When you persist with SDN, the returned object may not. What SDN does is save the data, then do a basic lazy fetch resulting in a new object instance. Which means that unless you annotate your complex properties with “@Fetch”, you’ll end up with a nasty surprise. Unlike JPA, which returns either the original value or an unresolved proxy object, SDN returns null for unresolved lazy fetches. Which is probably a recipe for lost data and is definitely a recipe for confusion (remember, this is things I learned the hard way!)
Ouch!
You can code @Query on both Repository and EntityNode classes
The manual doesn’t mention this. It’s a useful thing to know, although since in EntityNode classes, you often want to base your query relative to the current EntityNode instance, I also spent a lot of time coding things like “{self}” and “{this}” without success. I finally found out that using the clause START node=({self}) does the trick, although since “START” is supposed to be deprecated, there’s likely something that can do the same thing with a simple MATCH. That’s something to tackle another day.
What you get back isn’t what you think it is
Cypher isn’t as intuitively obvious to me as some people seem to think it should be. It’s a form of complex mathematical notation and while the docs on it are fairly illustrative, they are perhaps less complete or explanatory as they might be. So I spent a lot of time trying weird expressions. Here’s what worked as a member query on my “B” node:
@RelatedToVia
List wantsList = new ArrayList(5);
@Query("START b=node({self}) MATCH (b)-[r:WANTS]-(a:A) return r ORDER BY r.priority")
public List getWants() {
return wantsList;
}
A lot of the earlier attempts returned false “Wants” objects whose nodeIds were actually the IDs of the “A” objects on the other end of the relationship.
Incidentally, Neo4j normally wants relationships to be unordered and will complain if you attempt to use an ordered collection (such as List) to hold them. However it is smart enough to realize that when you do an “ordered by” query it’s OK to define the collection using an ordering collection object.
But that’s not all!
The “Wants” list that this particular query returned is not directly usable. The nodeIds are valid, but all the other property values were blank. Not merely the lazy-fetch values, but even the primitive property values. So to be actually usable I have to pull the nodeId from the returns pseudo-Wants and use the repository to lookup the actual Wants node.
So I’ve solved what should have been a relatively simple problem – even though it’s an ugly solution – and only lost about 2 days doing so.