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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