Perl’s killer feature – DBIx::Class::Schema::Loader

After I tell people I’m a programmer and they ask ‘which language’, they’re usually pretty shocked when I tell them that for server-side I use Perl out of preference. I guess most people have seen some really bad scripting in perl patched together by successive non-developers at some point in their life and assume that’s what I mean, but that’s a story for another day.

Anyway, one of the main reasons I use perl as my language of choice is because of the excellent ORM, DBIx::Class which allows you to do a lot of complex database queries and joins without having to touch SQL. Fine, you can get a decent Object-Relational Mapper (ORM) in many different languages – PHP, ruby, python, node.js for example. DBIx::Class does seem to have a lot more databases supported than your typical backend though. However the killer feature is not DBIx::Class but a separate module, DBIx::Class::Schema::Loader.

Here’s the beauty of DBIx::Class::Schema::Loader – with a typical ORM you need to define every table’s schema and relationships in code before you can start using it fully, however this module goes into the database and pulls out all of the tables, indexes, relationships and creates the full set of classes which you are then free to edit or update as you wish. The next time you run it it will pull in any database schema updates and keep the custom stuff you wrote. I’m yet to see this functionality in any other language – please point it out to me if it exists.

Now I know at this point there are many devs who like to define a database schema in code and then push it to the database so you can have versioning etc (which by the way these modules also support if you want to use them that way round). Whilst that might be OK for a new project, it’s a nightmare for an existing database or project – just think about how many days work would it be to build up ORM models of 100 tables manually. Also in my experience writing schemas in Ruby, Node.JS or even DBIx::Class is pretty nasty and repetitive – you can do a lot better using SQL CREATE TABLE statements. Another issue is that it’s very difficult to define indexes properly – perhaps you can define an index using the ORM schema definition language, but I’d wager most ORMs don’t know much about how to create a partial or functional index for example in postgres you can easily specify complicated indecies for example:

or

Certainly once you go into the realms of complex geo-spatial data-types it’s impossible not to touch the database manually no matter what language or ORM you use.

An example of using DBIx::Class::Schema::Loader to connect to a database and dump all of the schema we simply have the following as a library (under dbic_lib/Parts/Schema.pm):

And as the dumper script:

That was easy wasn’t it? This works whether you have 2 tables or 200. So far I’ve used it with MySQL, PostgreSQL, MSSQL, Oracle, SQLite and it works brilliantly in all of these thanks to the excellent driver support in DBI which all of this is based on.

In a previous job we had an MSSQL data-warehouse that we wanted to connect to and pull out various views that defined customer lists so could email them. We used this module to refresh the schema dump on demand, reload it into the app and allow people using just a web browser to choose a view from the data warehouse, define columns or search criteria put these into a template (also created by them on the frontend) and email out. All this in about 30 lines of perl code for the database interface.

As another example just this week I was given the task of figuring out a client’s database structure with over 150 tables only about 5 of which even had a primary key. First thing – whip out the code above and get a full dump of all the tables into a perl class structure (by the way it was an oracle database – I’ve never used that brand of SQL before, wouldn’t know what command-line linux client to use and it’s a bit strange it doesn’t even have LIMIT – not a problem for DBIx::Class though). Within about 5 minutes I had a full dump of all tables including indexes, column types (and relationships if any had been defined). Within about 10 more minutes I could write a script to dump complex data based on relationships between the tables into a JSON format.

Please show me another language that has such great database-agnostic ORM support.

Update: DBIx::Class::Schema::Loader comes with a tool called dbicdump. If you just want to take a simple dump of the schema of the database above rather than writing the dump script yourself you can just run:

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