[elephant-devel] Representational Question

[Leslie P. Polzer]

> Since we all have a very strong and hard-headed background on MySQL
> and relational models, it's been extremely difficult for us to migrate
> away from that mentality and think of objects and some of Elephant's
> terminology such as class indexes, which kind of confuse us into
> thinking that a class index allows us to look at a set of objects in a
> similar way as a MySQL table.

No. A class itself does that. The indexing mechanism is just a way
of automatically storing every new instance and gathering those
instances.

A class slot index is actually pretty much the same as a column index
in SQL.


> I've read and seen in the src the beginning efforts to building a
> query system into Elephant. That would be great and as our efforts
> approach that phase, we hope to contribute to it.

You can use MAP-* with appropriate functional queries until then.
It's different from SQL queries. More than one leven of sorting
would have to be implemented by you.


> So, in this email, first I will ask for advise as to how to best
> represent the structure of our objects/classes and indices in Elephant
> in order to ultimately be able to query the data. Again, I'm not going
> to ask for the querying strategy (just yet) but ultimately, we will
> need to be able to answer queries like this. Obviously I don't expect
> anyone to give me the full representation of this, but any advise/
> hints as to best represent them will help greatly.

Looks like you could make use of some professional consulting here.


> The way we see it, the concept of tables disappears

As such yes. But you can model classes with tables and vice versa.


> and so do the
> tables that provide many-to-many joins. So, we end up with some
> classes such as "Person" which contains a reference to a list of
> "Address" objects, and a list of preferred "Medical-Office" objects,
> where each Medical-Office object has a list of Doctor objects and each
> Doctor has a list of Specialty objects, etc, etc.

Yes.


> Now, we assume that each of these classes will need to maintain
> multiple indices, such as the Person class being index on first name,
> last name, dob, gender, among others. The Address class indexed on zip
> code, county name, among others, and so on and so forth.

If you like it simple (and non-performant), you don't need any indices
at all. Just use MAP-CLASS.


> The querying is one problem. The data representation is another. We
> think it's clear that we should have, as an example, a Person class.
> However, the representation of the links between a Person and its
> Addresses or Medical-Offices is not 100% clear. If we represent them
> as a slot in the Person class, where this slot would be a List or a
> set of references to the Address class, then in order for us to query
> on those, means that we always need to fetch all objects in those
> slots in order to apply any search criteria, which seems like a
> bottleneck. If that was the solution, I assume we could implement
> logic such that Addresses are pushed into the list, so that the most
> recent address is in the CAR, so we wouldn't necessarily need to read
> the entire list of Addresses for each member, but just fetch the CAR
> of the slot.

Now here you should use btree indices [1].


> Now, onto the second question.

Yuk. Seems there's a bunch of separate issues in this "second question".
I'll try to handle them separately.


> One of the other requirements we have
> is that we need to keep an audit log of data changes.

You can have a simple log with some trivial MOP changes.


>  From what we've read in Elephant's manual, this seems harder because
> we don't want to work directly off the Elephant object but a memory
> copy while the user takes his/her time in the browser and after
> submitting, we would take the changes and commit them to the Elephant
> object.

Sounds sensible, yes.


> Makes me think that we would need to classes for each object
> (one with and one without the persistent metaclass).

No, not necessarily. New instances only get added to the persistent
storage when they are of indexed classes. Otherwise you need
an explicit ADD-TO-ROOT.

If you need both indexing and this fine-grained behaviour,
use your own indexing stuff with non-indexed objects.


> The other problem
> would be how to "easily" have two objects introspect themselves and
> spit out the slots that changed between the two.

Use the MOP facilities.

An alternative solution to the whole problem is off-loading this
to the client. Have JavaScript record which fields changed.

  Leslie


> Are we looking at this incorrectly? Any advise would be greatly
> appreciated.

You seem to be a bit confused. Try hands-on experiments.

  :) Leslie


[1] http://common-lisp.net/project/elephant/doc/BTree-Indexing.html#BTree-Indexing