On 10 Sep 2009, at 16:08, Jos Elkink wrote:
> This can become quite a long list. Since they are single table
> inheritance, all the information ends up in one SQL table. I.e.
> notes have titles and contents, which means that now every item
> (incl. e.g. knives) have a "title" and "content" field. Not in the
> class definition, of course, but in the SQL table.
>
> My simple question is: is this bad design? Does this lead to super
> inefficient database usage? Or is this just fine and common?
>
> In terms of practical usage and coding, the inheritance definitely
> brings substantial benefits.
Hi Jos
Personally, I follow Date on this, and try to avoid NULLs at every
step. Occasionally they are necessary, I can think of one nullable
column in my current project, although there may be others I forgot
about. Having excessively wide tables may or may not be a performance
issue, depending on the DBMS you're using. A bigger issue I've found
is enforcing integrity, eg making sure Knife objects don't end up with
a note title.
I've been forced to use STI to solve a problem on this current
project. To avoid making non-sensical database tables, you can keep
only the core properties on the STI table, and use associations to
reference the extra properties. eg you might have
class Note < Item
has 1, :note_description
end
class Knife < Item
has 1, :knife_blade
end
etc. Although watch out, there are still a couple[1][2] of issues
with STI. That said, it can make some problems much easier to solve.
But treat large class hierarchies as a warning sign - if you have too
much inheritance, you may be coupling some of your code too tightly.
(Again, my personal preference is to never use inheritance, but
occasionally it is the most pragmatic solution.)
HTH
Ashley
[1]
http://datamapper.lighthouseapp.com/projects/20609/tickets/1001-sti-broken-when-subclassed-parent-to-a-child-possibly-connected-in-some-way-to-validation-code
[2]
http://datamapper.lighthouseapp.com/projects/20609/tickets/1002-sti-association-not-accessible-on-certain-reloaded-resources
--
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http://www.linkedin.com/in/ashleymoran
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