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From Christophe Lombart <christophe.lomb...@gmail.com>
Subject Re: OCM - Inheritance & Interface
Date Fri, 10 Feb 2006 13:14:50 GMT
Excellent ! No comment until now.

This proposal contains nice flexibilities to solve different uses
cases. I like the option to use the discriminator property or the node
types. It should cover 90% of the inheritance issues.
You are right, we found a lot of inspiration from tools like Hibernate
and OJB. the class-descriptors, ... comes from OJB, the same for
auto-retrieve, auto-update  proposal  :-)

Now, what about the interface support ? Do you want to speak about
that now or later ?


On 2/10/06, Alexandru Popescu <the.mindstorm.mailinglist@gmail.com> wrote:
> Hi!
>
> Here is my proposal for handling inheritance:
>
> 1/ Node type per class hierarchy
>
> Scenario: all hierarchy classes are using the same node type for persistence. This strategy
is
> covering scenarios where nt:unstructured or some other very generic node type is used
for persisting
>   a hierachy of classes.
>
> Inheritance behaves normally: all subclasses are inheritting properties/nodes from their
parent classes.
>
> Note: To differentiate between the classes the mapping should define a discriminator
property: a
> property that determines what the real type of the corresponding object in hierarchy.
Without this
> requirement, queries cannot be performed.
>
> Constraint: if the node type has mandatory properties/nodes, than these must be mapped.
Otherwise,
> the storage will not work.
>
> Example:
>
> As a quick example thing about the scenario where People is extendend by Student and
Teacher
> classes, and both Student and Teacher are persisted using a node type: nt:people.
>
> 2/ Node type per subclass
>
> Scenario: each non-abstract subclass is using its own node type for persistence. This
strategy is
> covering scenarios where very specific node types are used for mapping real classes.
>
> Details: there are no visible problems/constraints regarding this scenario. Inheritance
behavses
> normally: all subclasses are inheritting properties/nodes from their parent classes.
>
> Constraint: the parent node under which the subtree of these classes is hosted must be
generic
> enough to support different node types (f.e. nt:unstructured).
>
> 3/ Implicit/Explicit polymorphic queries
>
> As Marcel Reutegger (Jackrabbit committer) pointed: according to section 6.6.3.2 of the
JCR spec:
>
> A type constraint specifies the common primary node type of the returned
> nodes, plus, possibly, additional mixin types that they also must have.
> Type constraints are inheritance-sensitive in that specifying a
> constraint of node type X will include all nodes explicitly declared to
> be type X, as well as all nodes of subtypes of X.
>
> Translating this in Java hierarchies:
>
> when searching for an object type X, than the query will return by default all nodes
of type X or
> any subtype of X.
>
>
> For the "node per class hierarchy" strategy this will use the discriminator property
value to
> instantiate the correct subtypes of X. Without the discriminator property, there is no
possible
> solution to identify the correct subtype of X to be instantiated.
>
> For the "node per subclass" strategy this will use the union of all node types corresponding
to the
> X and all its subclasses.
>
> Note: the same strategy can be applied to search using interfaces.
>
> Explicit polymorhic query:
>
> However, there are cases when explicit polymorphic queries should be used. In this case,
querying
> for a type X will return only nodes corresponding to X class, disregarding any nodes
that map to a
> subclass of X.
>
> For the "node per class hierarchy" strategy this would suppose searching for nodes type
> corresponding to the specified hierarchy and for discriminator="condition to match only
type Y"
>
> For the "node per subsclass" strategy this would suppose searching for nodes having the
node type
> exactly matching the class, dismissing any nodes that correspond to possible subtypes.
>
> Supporting all these behavior in mapping would imply:
> - creating special mappings support for the 2 strategies (node type per class hierarchy
and node
> type per subclass)
> - adding an attribute specifying what is the polymorphic behavior: implicit or explicit.
>
> In my opinion this proposal covers a wide range of scenarios, hopefully all. Please take
your time
> to think about it and feel free to find missing points :-).
>
> cheers,
>
> ./alex
> --
> .w( the_mindstorm )p.
>
> ps: I must give credit for the inspiration and some good ideas to Hibernate guys and
all other
> contributers to ORM tools.
>


--
Best regards,

Christophe

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