MetaModelObject already has a getter (without using the word "get") to access
the node.
Wrapping MetaModelObject to give it a Node-interface makes it possible to
directly write:
metalObject.setProperty("a", b)
instead of
metaobject.node.setPropert("a", b)
If that were all, I wouldn't make a post about it. The more interesting part is
the meta modeling of the node gotten from a MetaModelObject. This node
represents the class, but it is not an instance of any class itself. That's
where the reify method comes into play, which takes the node from a
MetaModelObject and creates a class with the same name, but in a different
namespace, and makes the node an instance of this new class.
With that construction it becomes possible to model the
relationships/properties of a class.
There are many examples where this can be handy.
I already gave the example of HTML tags, where the attributes can be modeled as
properties and the tagname as a property of the meta class.
Another example is:
All countries have subdivision. Countries and their subdivisions all are
instances of a certain class. There is one class for country, but there is a
set of classes for subdivisions. In some countries, a subdivision is called
"province", in others it's a "state" or a "district". Subdivision of various
countries can have different sets of properties and relations.
How to model the fact that the instance "United States of America" has
subdivisions of the class "State_(US)"? Using the reify method we can make a
class "United States of America", where we can model that the ClassRange of
"State_(US)" is "United States of America". Or if we want the relationship to
point the other way, that the "United States of America" has 50 (use of
cardinality) subdivisions of the class "State_(US)".
Of course all this can directly be expressed with nodes and relationships, but
that's what the meta model does anyway.
I do have one peeve with the meta model API. The class DataRange has a
constructor:
DataRange(String datatype, Object... values)
I'd much rather see a new Restrictable class Datavalue, and see a constructor:
DataRange(String datatype, Datavalue... values)
That way the possible values a property can have additional properties and
relationships (eg. link to Wordnet definition or Wikipedia entry).
Kind regards,
Niels Hoogeveen
> Date: Tue, 30 Mar 2010 09:30:10 +0200
> From: [email protected]
> To: [email protected]
> Subject: Re: [Neo] meta meta classes
>
> Would making the underlying Node publically available (via a getter) be
> virtually the same thing? In that case the meta model classes could have
> such a getter.
>
> 2010/3/26 Niels Hoogeveen <[email protected]>
>
> >
> > Hi Peter,
> >
> > I added a Wiki entry in my github repo called "Reification of meta classes
> > and meta properties":
> >
> >
> > http://wiki.github.com/NielsHoogeveen/Scala-Neo4j-utils/reification-of-meta-classes-and-meta-properties
> >
> > The source code for the Scala wrappers can be found found in my repo:
> >
> > http://github.com/NielsHoogeveen/Scala-Neo4j-utils
> >
> > Kind regards,
> > Niels Hoogeveen
> >
> > > From: [email protected]
> > > Date: Fri, 26 Mar 2010 17:25:01 +0100
> > > To: [email protected]
> > > Subject: Re: [Neo] meta meta classes
> > >
> > > Awesome Niels!
> > >
> > > maybe you could blog or document some cool example on this?
> > >
> > > Cheers,
> > >
> > > /peter neubauer
> > >
> > > COO and Sales, Neo Technology
> > >
> > > GTalk: neubauer.peter
> > > Skype peter.neubauer
> > > Phone +46 704 106975
> > > LinkedIn http://www.linkedin.com/in/neubauer
> > > Twitter http://twitter.com/peterneubauer
> > >
> > > http://www.neo4j.org - Your high performance graph database.
> > > http://www.tinkerpop.com - Processing for Internet-scale graphs.
> > > http://www.thoughtmade.com - Scandinavias coolest Bring-a-Thing party.
> > >
> > >
> > >
> > > On Fri, Mar 26, 2010 at 5:22 PM, Niels Hoogeveen
> > > <[email protected]> wrote:
> > > >
> > > > Using Scala, I was actually able to extend MetaModelThing to act as a
> > Node and MetaModelClass to have shadowing functionality for both
> > MetaModelClasses and for MetaModelProperties, without touching the original
> > source code.
> > > >
> > > >> To: [email protected]
> > > >> From: [email protected]
> > > >> Date: Fri, 26 Mar 2010 14:29:03 +0000
> > > >> Subject: Re: [Neo] meta meta classes
> > > >>
> > > >> Such are the joys and challenges of frameworks and abstractions.
> > Sometimes you do need to get "close to the metal" though, to achieve
> > specific functional and performance requirements. Thus the reason open
> > source frameworks are awesome. At least we can change and extend them more
> > easily!
> > > >>
> > > >>
> > > >>
> > > >> -----Original Message-----
> > > >> From: Niels Hoogeveen <[email protected]>
> > > >> Date: Fri, 26 Mar 2010 15:20:15
> > > >> To: <[email protected]>
> > > >> Subject: Re: [Neo] meta meta classes
> > > >>
> > > >>
> > > >> Making MetaModelThing implement the Node interface is actually
> > orthogonal to the creation of Shadow objects. Though it does make code using
> > classes as objects cleaner, and allows the node property of MetaModelThing
> > to become private.
> > > >>
> > > >> > From: [email protected]
> > > >> > To: [email protected]
> > > >> > Date: Fri, 26 Mar 2010 14:38:08 +0100
> > > >> > Subject: Re: [Neo] meta meta classes
> > > >> >
> > > >> >
> > > >> > Or, and this probably the simplest solution. Let a MetaModelThing
> > implement the Node interface. This allows a class to be an instance of
> > another class and/or a SubType of another class.
> > > >> >
> > > >> >
> > > >> > > From: [email protected]
> > > >> > > To: [email protected]
> > > >> > > Date: Fri, 26 Mar 2010 14:20:07 +0100
> > > >> > > Subject: Re: [Neo] meta meta classes
> > > >> > >
> > > >> > >
> > > >> > > The class as object phenomenon is actually quite pervasive, and I
> > think it is possible to have a generalized solution for it.
> > > >> > >
> > > >> > > After sending this email yesterday, I came up with a slightly
> > different solution.
> > > >> > >
> > > >> > > val taggable = namespace.getMetaClass("taggable", true)
> > > >> > >
> > > >> > > val tagName = metaModelNamespace.getMetaProperty("tagname", true)
> > > >> > > tagName.setCardinality(1)
> > > >> > >
> > > >> > > val metaTaggable = metaModelNamespace.getMetaClass("taggable",
> > true)
> > > >> > > metaTaggable.getDirectProperties.add(tagName)
> > > >> > > metaTaggable.getDirectInstances.add(taggable.node)
> > > >> > >
> > > >> > > val body = namespace.getMetaClass("body", true)
> > > >> > > taggable.getDirectSubs.add(body)
> > > >> > >
> > > >> > > Instead of directly relating the class "body" to the meta class
> > "taggable", I create a shadow of the meta class "taggable" with the same
> > name in the namespace of the class "body", and make "body" a subclass of
> > this shadow. That way the sub classing relationship remains nicely in one
> > name space, while the instance relationship transcends name spaces, as it
> > should.
> > > >> > >
> > > >> > > This could in principle be generalized by adding a methods to
> > MetaModelClass:
> > > >> > >
> > > >> > > public MetaModelClass createShadowClass(NameSpace ns){
> > > >> > >
> > > >> > > MetaModelClass mc = ns.getMetaClass(this.getName(), true);
> > > >> > > this.getDirectInstances.add(mc.node);
> > > >> > > return mc;
> > > >> > > }
> > > >> > >
> > > >> > > This returns a shadow of a given MetaModelClass in a given
> > namespace and adds it as a an instance of this.
> > > >> > >
> > > >> > > It would of course be nicer to have a method with the signature:
> > > >> > >
> > > >> > > public MetaModelClass getShadow(NameSpace ns, Boolean create)
> > > >> > >
> > > >> > > This is much more in line with the rest of the API, but requires a
> > way to find out the namespace a given MetaModelClass is defined in. I didn't
> > see a method getNamespace() for a given class, and didn't delve deeply
> > enough in the source code to figure out how to do that.
> > > >> > >
> > > >> > > A similar approach can of course also be applied to
> > MetaModelProperties, by adding the following method to MetaModelClass:
> > > >> > >
> > > >> > > public MetaModelProperty
> > > >> > > createShadowProperty(NameSpace ns){
> > > >> > >
> > > >> > >
> > > >> > >
> > > >> > > MetaModelProperty mp =
> > > >> > > ns.getMetaProperty(this.getName(), true);
> > > >> > >
> > > >> > >
> > > >> > > this.getDirectInstances.add(mp.node);
> > > >> > >
> > > >> > > return mp;
> > > >> > >
> > > >> > > }
> > > >> > >
> > > >> > >
> > > >> > > This way the underlying node of a MetaModelProperty can properly
> > be used as a class of its own and have properties, relationships, that can
> > be further modeled in the meta layer. This e.q. allows to set a default
> > rendering format for a given property class.
> > > >> > >
> > > >> > > I don't see a MetaModelRelationships, which is unfortunate, since
> > that would allow the modeling of the properties of a Relationship.
> > > >> > >
> > > >> > >
> > > >> > > Kind regards,
> > > >> > > Niels Hoogeveen
> > > >> > >
> > > >> > >
> > > >> > > > Date: Fri, 26 Mar 2010 11:26:29 +0100
> > > >> > > > From: [email protected]
> > > >> > > > To: [email protected]
> > > >> > > > Subject: Re: [Neo] meta meta classes
> > > >> > > >
> > > >> > > > That's an interresting case you've got here and it looks to me
> > like it's
> > > >> > > > probably the best way to model it in the meta model.
> > > >> > > >
> > > >> > > > 2010/3/25 Niels Hoogeveen <[email protected]>
> > > >> > > >
> > > >> > > > >
> > > >> > > > > For my application, I want to model an HTML template in Neo4J,
> > using the
> > > >> > > > > MetaModel api.
> > > >> > > > >
> > > >> > > > > So I started setting up MetaModelClasses for the various HTML
> > entities.
> > > >> > > > >
> > > >> > > > > e.g. (code in Scala)
> > > >> > > > >
> > > >> > > > > val classProp = namespace.getMetaProperty("class", true)
> > > >> > > > > val idProp =
> > > >> > > > > namespace.getMetaProperty("id", true)
> > > >> > > > > idProp.setCardinality(1)
> > > >> > > > >
> > > >> > > > >
> > > >> > > > > val body = namespace.getMetaClass("body", true)
> > > >> > > > > body.getDirectProperties(classProp)
> > > >> > > > > body.getDirectProperties(idProp)
> > > >> > > > >
> > > >> > > > >
> > > >> > > > > Which creates a class named "body" which has a property named
> > "class"
> > > >> > > > > without a cardinality restriction and a property named "id"
> > with a
> > > >> > > > > cardinality restriction of 1.
> > > >> > > > >
> > > >> > > > > Now I can create nodes of class "body" with various values for
> > "class" and
> > > >> > > > > for "id".
> > > >> > > > >
> > > >> > > > > So far so good, but now I want to say that the class body has
> > a property
> > > >> > > > > "tagname" which should get the value "body".
> > > >> > > > >
> > > >> > > > > The meta model itself doesn't allow classes to have
> > properties, but it
> > > >> > > > > allows access to the underlying node of the class.
> > > >> > > > >
> > > >> > > > > So I created a separate namespace for the meta meta classes
> > and added the
> > > >> > > > > following three definitions:
> > > >> > > > >
> > > >> > > > > val tagName = metaNamespace.getMetaProperty("tagname", true)
> > > >> > > > > tagName.setCardinality(1)
> > > >> > > > > val taggable = metaNamespace.getMetaClass("taggable", true)
> > > >> > > > >
> > > >> > > > > body.node.setProperty("tagname", "body")
> > > >> > > > >
> > > >> > > > > This creates a property "tagname" with the value "body" for
> > the class named
> > > >> > > > > "body".
> > > >> > > > >
> > > >> > > > > Now comes the more ambiguous part, how to link "body" to
> > "taggable".
> > > >> > > > >
> > > >> > > > > I can make body.node an instance of taggable. Once there is a
> > validator,
> > > >> > > > > the existence of a "tagname" property with cardinality 1
> > should be checked,
> > > >> > > > > so it is reasonable to make body.node an instance of taggable.
> > > >> > > > >
> > > >> > > > > At the same time the class body is actually a subclass of
> > taggable, so I am
> > > >> > > > > inclined to define that as well.
> > > >> > > > >
> > > >> > > > > So I end up doing the following:
> > > >> > > > >
> > > >> > > > > taggable.getDirectInstances.add(body.node)
> > > >> > > > > taggable.getDirectSubs.add(body)
> > > >> > > > >
> > > >> > > > > I would like to know if this is the correct approach to this
> > situation, or
> > > >> > > > > whether there are better alternatives.
> > > >> > > > >
> > > >> > > > > Kind regards,
> > > >> > > > > Niels Hoogeveen
> > > >> > > > >
> > > >> > > > >
> > > >> > > > >
> > _________________________________________________________________
> > > >> > > > > New Windows 7: Find the right PC for you. Learn more.
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> > > >> > > > >
> > > >> > > >
> > > >> > > >
> > > >> > > >
> > > >> > > > --
> > > >> > > > Mattias Persson, [[email protected]]
> > > >> > > > Neo Technology, www.neotechnology.com
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>
> --
> Mattias Persson, [[email protected]]
> Neo Technology, www.neotechnology.com
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