Hi Waldo,
Why do you want to migrate to Elephant for production and not stick
with something like CL-SQL or cl-perec on top of a relational database
so you get all the facilities that you're familiar with?
Also, please don't expect a query system anytime soon. Finishing it
is not in my critical path right now and no one else has stepped up
and volunteered to lead or help with it.
As for your query problem, I think the SQL solution for queries like
that is likely to be faster in the end than putting this into
Elephant. Elephant is not intended or designed to support efficient
relational operations. That's what relational DBs are for! :)
Wait until the next big update to elephant before you go too far down
this road, I'm hoping that some new features I'm planning at least
make this a little bit easier.
For your second question, if you are going to save/store the objects
in bulk, you can just use standard classes. Then you can have a
transaction to fetch/diff/write the composite object to ensure
atomicity of updates. This diff would also produce your log. However
that means that you lose the indexing capability of persistent objects.
Ian
On Feb 29, 2008, at 2:47 PM, [EMAIL PROTECTED] wrote:
Hi all,
As I'm further exploring more and more things to do in Elephant and
Lisp, I think we're ready to start migrating some of our RoR apps
over, if not just as an exercise, we'll someday migrate them to
production.
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.
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.
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.
We have a database with many related tables. For simplicity
purposes, we'll describe a simplified scenario. We have a table with
people information (e.g. first,last names, date of birth, and
gender). We have a linked table with each person's addresses
(multiple addresses in case they moved. Each address is timestamped
so the most recent address is the current address). Then, each
person may be subscribed to one or more health insurance plans, and
so there is a table linking each person to one or more health
insurance plans (and a table that defines the health insurance plans)
Now, each person may select up to N preferred medical offices where
they would like to receive treatment. Again, there is a table that
links the person with one or more medical office. Needless to say,
there is a table of medical offices. Each medical office is also
linked to a timestamped address table, where the most recent address
is the current one (in the event the office moves). To further
expand on the issue, each office has one or more doctors rendering
services, so there is a table that links the offices to the doctors,
and of course, there is a table of doctors that contains basic
information, such as fname, lname, and gender. Last, but not least,
a doctor may be specialized in multiple areas, so there is a table
that links doctors to all the specialties they have been certified
on, and thus there is yet another table that lists all possible
specialties.
Now, assuming I was able to explain the scenario correctly, we then
have users asking the system for information such as:
"List all people (subscribers), who are male and live in zip code
33012 who are contracted under Health Insurance Plan A that have
selected (as their preferred medical office) medical offices with
male cardiologists that work within 10 miles of 33012 zip code or in
MIAMI-DADE county and whose office names contain the sequence of
letters 'HEAL'"
The way we see it, the concept of tables disappears 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.
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.
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, onto the second question. One of the other requirements we have
is that we need to keep an audit log of data changes. The way we do
it in RoR is relatively simple. We fetch an object from the DB and
present it on the browser. When the user submits, we fetch another
fresh copy from the DB and if the timestamps are the same (meaning
no one else changed the record) we compare changes to the object's
attributes (slots). If there are any differences, we save the
changes (we're trying to avoid unnecessary trips to the DB) and if
the changes are saved successfully, we write a log of ONLY the
attributes that were changed (which is pretty trivial in Ruby).
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. Makes me think that we would need to classes for
each object (one with and one without the persistent metaclass). The
other problem would be how to "easily" have two objects introspect
themselves and spit out the slots that changed between the two.
Are we looking at this incorrectly? Any advise would be greatly
appreciated.
Thanks,
Waldo
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