[elephant-devel] Representational Question

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Hi Ian,

Thanks for the prompt response. I know the querying facility is not  
necessarily a priority at this time, but will someday become a  
reality :)

To tell you the truth, we haven't really had any direct experience  
with Elephant in production or larger-scale type projects. However, we  
do feel that the whole concept of object prevalence given the  
complexity of the overall data model would make Elephant a more  
appropriate framework than continuing the relational path (maybe we're  
just wrong and Elephant is not best suited for this at all). As it is,  
we currently need to do a lot of work to maintain all the data  
relations and integrity in the current system and hopefully working  
only with the object models would make things easier and more  
"maintainable". Granted, I agree that at this moment, it's a lot  
easier to formulate those queries in SQL, but I'd like to at least be  
able to setup a parallel model and migrate data over so we could  
compare performance (we're not even going to talk about the complexity/ 
difficulty of querying in Elephant, since we know that at this stage,  
it is much more complex than SQL queries).

I hope I'm not wrong, but definitely your opinion is worth more since  
you (et al) know a lot more about this than us.

As for the second question, the answer is no. The objects would not be  
stored in bulk. The idea is to keep an audit log of user-initiated  
changes on individual entities (e.g. changing a Person's address, or  
correcting a name, or assigning a health insurance plan, etc).

Thanks,
Waldo

On Feb 29, 2008, at 3:36 PM, Ian Eslick wrote:

> 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, lists at infoway.net 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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