Tuesday, December 02, 2008

Open Source fragmentation and cognitive biases

I've just finished Predictably Irrational (it was nice, many of the of the theses I already knew from Internet articles - so it was not ground-breaking for me, but still I learned from it). In the seventh chapter of this book entitled "The High Price of Ownership" the author presents some surprising stories about how we "overvalue what we have". Apparently we all give much more value to things that we posses then to those that we could potentially have - this is a common cognitive bias.

This made me thinking. Wouldn't that explain the fragmentation of Open Source libraries (and CPAN in particular)? Would that not explain why one author values the miniature efficiency gains of his reinvented wheel over the robustness of time-tested code?

Maybe once we know how this mechanis works, we could design better Open Source library repositories, limitting the power of that fragmentation drive? Maybe by being more explicite about our choices we can become more rational?

Thursday, October 23, 2008

Catalyst helpers - plugins for Module::Starter?

Code generation is hard. First of all it is hard to determine what really needs to be created - in (computing science) theory you can always uncurry the generator and use it as a library. So you need different theory to really determine if code generation makes sense at all. In practice many people use it - since it let's us start new projects faster (and more correctly). This is an interesting subject deserving many more blog posts - but for now I just wanted to toss the idea that joining forces with other Perl projects facing this problem could help us improve the mess that is Catalyst helpers now.

I've learned about Module::Starter plugins from Recursive development that leads nowhere.

Thursday, October 16, 2008


Now that I use CGI::Application, Class::DBI and HTML::Template at work I can see the advantages of using Catalyst, DBIx::Class and Template Tookit. Personally, I find using HTML::Template as the most annoying of those three set backs - with TT I've got used that I don't need to worry about displaying the data I feed to the template, I need to make sure that it contains everything needed but that's all. I know that the view can take any data and display it. This makes a nice separation of work and of mind sets between the Controller and the View. Not so with HTML::Template.

Thursday, August 28, 2008

One gotcha less!

Looks like one of the anomalies I described in my previous post is fixed now in the svn (see Revision 4773 of DBIC). Thumbs up!

Friday, June 13, 2008

DBIx::Class gotchas

DBIC is a complex beast - and it has it's rather obscure corners where it can burn the inexperienced. Here are two of those, comments with others are welcome.

Don't use $rs->update_or_create(...) (or $rs->find_or_create) on a table with autoincrement primary key

The ->find method requires that all the columns comprising the key used for the search are present in the query or in the resultsource conditions. Otherwise it will return some random record (and emit a warning). This is because currently there is no way to determine if all the needed columns are present. To do this you would have to parse the arbitrary SQL::Abstract query that can be in the resultsource condition and check how the columns are used there. Think for example about:
$new_rs = $cd_rs->search( { -and => [ id => [1, 2], id => { '>' => 1 } ] } )
and then $new_rs->update_or_create( { artist => 1 } ).

But for autoincrement primary keys the ->create method requires that the primary key is not present in the query. If you provide { pk => 'undef' } to ->create, this would generate a query like: INSERT INTO cd ( pk, ...) VALUES ( NULL, ... ) - some databases would accept that but others (PostgreSQL for example) not.

So in summary, for tables with autoincrement primary keys ->find requires that you provide the primary key in the query, and ->create requires that you don't - this means that you cannot pass the same query hash to both methods and this is what ->update_or_create does.

Update: Looks like the 4773 revision fixes the problem described below. That's why I moved this point to the end of the post. I did not check it too deeply though.

Don't call $row->some_relation when the $row has not loaded the columns required to resolve some_relation

Column values in DBIC are kept in a hash - this means that a column can have some defined value, have undefined value or can be completely non-existent - and DBIC interprets all of those states differently. When $row->some_relation is called and ->some_relation uses a foreign key column that is non-existent then the result is currently random (and in the future most probably an exception will be raised).

When this can happen? When you create a row and don't provide the value for the column (or if you retrieve the row from the database and explicitelly omit the column). For example if you do
$cd = $cd_rs->create( { name => "Cisza" } )
(or $cd = $cd_rs->new...). Now the 'artist' column in $cd is non-existent and when you try to call $cd->artist it blows up.
In a library where you don't know how a record was created you need to check first if the relevant columns are loaded before you call a relation on the record.

I thought that a good remedy would be also to always provide undef in the ->create and ->new calls for all the columns that we don't have values for, but allegedly this is not a good strategy (don't ask me why).

Tuesday, May 27, 2008

Installing Test::WWW::Mechanize::Catalyst

It seems that there is some incompatibility between the latest LWP version (5.812) and Test::WWW::Mechanize::Catalyst. The tests die with an:

HTTP::Message content must be bytes at lib/Test/WWW/Mechanize/Catalyst.pm line 88

error message. After downgrading LWP to 5.808 Test::WWW::Mechanize::Catalyst (version 41 - which looks better than 42 at CPAN Testers: Reports for Test-WWW-Mechanize-Catalyst) worked. Looks like this change is the culprit: "Don't allow HTTP::Message content to be set to Unicode strings".

confound - thanks for the hint.

Sunday, May 25, 2008

Interesting articles from the Ruby community

Why coupling is always bad / Cohesion vs. coupling and Why Rails is total overkill and why I love Rack - looks like the Catalyst philosophy is quite close to that of the Rack project (but maybe less articulated).

As a side-note - I cannot comment there (even though I have javascript turned on) - do you get the same problem?

Tuesday, March 04, 2008

Writing Facebook apps in Catalyst

A few weeks ago I've started a howto page for writing Facebook applications in Catalyst at the Catalyst wiki. Now I have accumulated there a few tips. Everyone using Catalyst for Facebook apps is invited to add their problems and solutions.

Wednesday, January 30, 2008

Refreshing a row from the DB - another note to myself

This is another thing that I keep forgetting - how to refresh a row with values from the database.
So here it is:


It comes from DBIx::Class::PK (and not from DBIx::Class::Row where I usually start the serach).

Update: The core devs say that the new version of documentation will mention discard_changes in DBIx::Class::Row POD. Thanks.

Tuesday, January 29, 2008

Automatic DBIC schema creation from a database

This is something that I keep forgetting and then I have to wade through the docs again. And there are traps in the docs - for example SQLT can do the schema creation - but it does not build the relationships.

So here is the magic invocation for DBIx::Class::Schema::Loader which do create the relationships (if you use the right switch):

perl -MDBIx::Class::Schema::Loader=make_schema_at,dump_to_dir:./lib -e make_schema_at("New::Schema::Name", { relationships => 1 }, [ "dbi:Pg:dbname=foo","postgres" ])’

Saturday, January 12, 2008

A free implementation of Amazon SimpleDB

Here is a tip to Amazon - there should be a free implementation of SimpleDB. I am now considering using SimpleDB for a web application - but I would like to develop it on my desktop computer - without paying Amazon, without even connecting to Amazon. This might be a minor issue - the fee you would pay for developing the application on Amazon web services seem to be very small. But it would be so much more convenient: no fiddling with credit cards, no problems for developers with shaky internet connection and more compatibility with Free Software/Open Source development models where money is always a difficult subject.

For Amazon it would not mean much less income - since as I've said the fee for the development use of the services is rather small - but it would mean many more customers. Perhaps it would also mean some competition - but this would only validate the whole model and make it more suitable for big business which does not like so much dependence on one provider.