Patrick Doyle wrote:
> On Thu, Sep 17, 2009 at 9:14 AM, Marnen Laibow-Koser
> <[email protected]> wrote:
>>
[...]
>> Yes. And also constraints in the database.
> That's a new one for me...
Really? It may not be worth implementing complex constraints in the DB,
but simple ones are almost always a good idea. Remember, the database
is not dumb, and it will work best for you if you respect its
intelligence and allow it to reason as much as possible about the data
it contains. This is one reason that we use a database instead of a
flat CSV file or some such.
There is a tendency among some Rails developers to treat the database as
a dumb data store, but that is generally a bad idea. (Of course, the
opposite extreme -- putting *all* the app logic in the DB -- is just as
bad.)
> I presume there are ways to implement the
> same sort of validates_presence_of
Just declare the field not null.
> and validates_uniqueness_of checks
> in databaseland,
Declare a unique index. These two things are so basic that you really
shouldn't be doing database design if you don't know about them.
> but what about custom validations that require, for
> example, text be formatted a certain way,
This one may be more trouble than it's worth in the DB. More than that,
if you have such a constraint, it suggests that you are storing your
data improperly. The DB really should store data, not formatting
(unless the formatting *is* part of the data, as for example in a CMS
that stores HTML fragments in the DB). If you can say more about your
use case here, I may be able to provide more specific advice.
> or a product price must be
> at least a cent, etc...
This is easy with a check constraint. Unfortunately, Rails migrations
don't make check constraints easy, and there's no plugin to add this
feature AFAIK. One of these days I hope to extend
foreign_key_migrations so it supports check constraints. Till then,
you're stuck with raw SQL.
>
> I guess I'm asking what is the value added to constraints in a
> database that will only ever be accessed via the web application
> front-end?
You are asking the question backwards. First of all, you can't
guarantee that the DB will only ever be used through the Rails app.
What happens 5 years from now when the Rails app is retired and
rewritten from scratch in Lolcode on Kittehs (or whatever the hot new
technology is in 2014)? Surely you will still want to have access to
the old data. Your data will almost certainly outlive your application.
Second, remember that validations in the Rails app can be thought of as
"suggestions". They only work insofar as data in the DB *is* only
modified through the Rails ActiveRecord interface. It is still possible
to get bad data into the DB by bypassing the ActiveRecord interface
(which you could even do with a naked INSERT query right from the Rails
app!). The *only* foolproof way to prevent bad data from getting into
the DB is to supply the DB with the tools to keep it out -- and that
means constraints.
In other words, the DB constraints are the primary, foolproof means of
keeping bad data out of the DB. The Rails validations are simply a
layer on top of that, to prevent the DB constraints from being triggered
in the first place, and to implement complex constraints that would be
difficult or impossible to implement in the DB.
>
>>>
>>> One thing I've thought of would be to write a throwaway rake task
>>
>> This seems quite reasonable. �If it's a question of the data structure
>> changing and affecting validity, then it's probably a good idea to put
>> the validation routine right in the migration file.
> In this (um, hypothetical) case, there is no migration file. It's
> just been noticed after the fact that there is something wrong.
OK. Then a rake task, or simply running Model.all.select{|m| !m.valid?}
from the console, is probably the way to go.
>
> Fortunately, this is hypothetically a very small, custom application
> that is only used (currently) by a single user, so the effects are not
> terribly disastrous.
For now. :)
>
> --wpd
Best,
--
Marnen Laibow-Koser
http://www.marnen.org
[email protected]
--
Posted via http://www.ruby-forum.com/.
--~--~---------~--~----~------------~-------~--~----~
You received this message because you are subscribed to the Google Groups "Ruby
on Rails: Talk" group.
To post to this group, send email to [email protected]
To unsubscribe from this group, send email to
[email protected]
For more options, visit this group at
http://groups.google.com/group/rubyonrails-talk?hl=en
-~----------~----~----~----~------~----~------~--~---
