[Dbix-class] Re: Re: Re: Relations and inflation

Wade.Stuart at fallon.com Wade.Stuart at fallon.com
Mon Jul 2 20:32:01 GMT 2007

> Hi Jess,
> I'm quite willing to give back to the project in terms of editing
> documentation but thought this might be limited to editing, because
> it should be written from the perspective of someone who knows the
> "right" way to use the system or its core philosophy.
> I can't reply at length now since it is 1AM but will think about it
> some more. For a quick reply, yes I've read that page, the cookbook
> page, Relationships::Base, page, etc. and a number of times. But to
> me anyway it seems to assume a lot of knowledge, skips describing
> the foundation of what the module is intended to do, and skimps on
> the important parts. In order to get a feeling of which method to
> pick and how to practically use it you basically have to read many
> different documents over and over, pulling information in about
> other methods, and finally of course already know a lot of about
> what is really going on in the database. To me the module ought to
> be shielding you from having to think about it so much but that
> probably isn't realistic. To illustrate a couple points come to mind:
> The very helpful replies I received about "insane layout" (i.e.
> having an id in both tables pointing to each other), it is not
> obvious to even someone who has used databases and perl a lot for
> small systems it isn't sane. Of course if the two relationships get
> out of sync then you might get different data depending on which
> table you start from, hence the insanity. But very little info on
> designing ORM based systems with lots of objects and relationships.
> Also no information about Relationships and foreign relations. In
> fact I had been under the impression at least that mysql3 couldn't
> do foreign relations, and I'd decided to do it all using DBIx
> Relationships instead. The notion of foreign keys in the db being
> useful for maintaining integrity of data is not mentioned; it seems
> that a Relationship would do something like this but of course it
> isn't the same.
> Also check out this part of the FAQ you mention, it is quite
> confusing actually.
> "Relationships
>    ... define a foreign key relationship where the key field may
> contain NULL?
> Just create a belongs_to relationship, as above. If the column is
> NULL then the inflation to the foreign object will not happen"
> Now according to Matt's very helpful notes in the ML, allowing NULL
> in your db schema is used to tell belongs_to to be an optional
> Relationship, meaning that it will not enforce the belongs_to rule
> and autovivify. In other words use NULL so DBIx::Class::
> Relationships does not do inserts when it thinks that would be "the
> right thing". (or you can make a dummy record, but that's painful I
> But the above passage comes from the other direction, assuming first
> that the developer wants to allow a NULL, and then explains that
> allowing NULL will disable object inflation (which I did not know
> until I just checked now!). It doesn't say that it disables
> autovivification, even though anything that does an INSERT is to me
> a potentially naughty thing and deserves to be well-documented.
> Now we go back to the perldoc for the belongs_to method does not
> mention that object inflation will be disabled, rather saying "If
> the relationship is optional -- i.e. the column containing the
> foreign key can be NULL -- then the belongs_to relationship does the
> right thing. Thus, in the example above $obj->author would return
> undef. However in this case you would probably want to set the
> join_type attribute so that a LEFT JOIN is done, which makes complex
> resultsets involving join or prefetch operations work correctly. "
> This does not explain whether object inflation being "disabled" just
> means returning an undef if it's a NULL (which of course would be
> intuititive) or whether it means something like $listing->coupon
> will not return an object in any case if the field in question
> allows NULL. It also does not explain about from which side to make
> a Relationship (or both sides - no that's insane apparently, despite
> being logical in English anyway), though it is mentioned in the FAQ
> (regarding one-to-many) that "Currently these need to be set up
> individually on each side."
> Also the pod mentions left joins and right joins. As I understand it
> a normal join would only provide records that match across the two
> tables but a left join would also show items not matched in the left
> table, and similarly right would for the right table. But this is
> not explained, and it is not clear why an ORM would not handle this
> sort of thing for you, or how the left/right sql metaphor for tables
> translates into parent object -> child object metaphor of DBIx::Class.
> Something about usage of foreign relations in the db for integrity
> control would be helpful, and would explain the difference between
> what DBIx::Class::Relationships offers and what the db offers.
> I think it would be quite helpful if a design example similar to
> what I had posted is used (you can use my objects if you want). For
> one thing there is confusion among people who want to use an object
> oriented system but then have to build an sql table schema to fit
> it. They start thinking in an object oriented fashion. Then they
> have to translate it into tables in their head (probably writing sql
> create statement by hand or with a tool like phpmyadmin, though
> maybe they really should use DbDesigner, or maybe they should just
> stay away from mysql and go to postresql). Anyway once they have the
> tables they start using DBIx::Class and it seems to work nicely,
> except it fails sometimes, and they end up having to figure out the
> more esoteric parts of the module which are not as well documented.
> I think this tends to lead to spaghetti objects, insane layout, and
> fragile schemas.
> Well this turned out to be a long post. Sorry, and no offense meant
> at all, just that I'm willing to contribute to documentation in
> thanks for your support. So this is in response to how a non-dba
> person might see the module. I've used DBI and CDBI, etc. with Perl
> for many years but don't make my living as a database designer, and
> haven't needed to get high end database performance (my high
> performance mod_perl systems have used a Perl embedded db or a C++
> based system).

Ok,  only made it this far (can't wait for the "you have more time" version
=). Heres the short answer -- the docs have been written (mostly) from
developers intimate with the code and behavior.  They try to put themselves
in the position of "non-dba" users, but as you have pointed out sometimes
they gloss over things that may be tricky for new users.  You on the other
hand are one of those New Users(tm) and have run up against some things you
feel are mis-docmented or glossed over on the docs.  You are in a better
position to offer New Useresq docs for these areas,  submit doc patches and
if they are factually incorrect you will hear back from people.  If they
are broke from your perspective -- fix them from your perspective. You WILL
get feedback, and you will learn how to use the ORM quickly by doing this.
If I misread the post,  or if you changed the topic at the end I apologize.


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