[rspec-users] OT: AR Default Values

[Scott Taylor]

On Mar 4, 2009, at 12:16 PM, James Byrne wrote:

> This is sort of off topic but it arose from a cucumber test failure  
> and
> I am unsure how to address the underlying issue.
>
> I spent yesterday tracking down an obscure error that was exposed by a
> new cucumber test.  It turned out to be an improper default value
> assignment made in a controller which in turn proved dependent upon
> recent changes (Rails 2.2) made in how AR handles timezones.  It also
> turned out that the same error was made in another controller handling
> that same model.
>
> This led me to consider that the place to handle AR default values
> really should be somewhere else and the logical places are either in  
> the
> model or in the DBMS schema.  The problem with the latter is that  
> Rails
> migrations provide no accommodation for dynamic defaults like the
> current time, other than by recourse to explicit SQL.  In any case, a
> DBMS provided default value is not going to be available to a view  
> until
> after the new row is INSERTed and subsequently SELECTEed, which is too
> late to be of any use in the presentation of a "new"/"create" resource
> pair.
>
> Thus, I considered it best to handle this requirement in the model.
> There is a vast amount of information on this subject but it  
> appears, to
> me, inconsistent and the issue unresolved.  The obvious answer is to
> override the initializer method.  However, there are several warnings
> that basically say that for AR this is NOT A GOOD IDEA!  There are a  
> few
> references that indicate AR may not actually call #new in every case.
> There is also the matter of interpretation of what new actually  
> means to
> an ORM.  A new model instance is not always the same thing as a new  
> DBMS
> "row".
>
> There are references to using the :after_initialize callback.   
> However,
> after_initialize is called on every row instantiation whereas a  
> default
> value only applies on a new DBMS instance.  This is an old topic for  
> the
> rails core team:
>
> http://groups.google.com/group/rubyonrails-core/browse_thread/thread/b509a2fe2b62ac5
>
> So, after confusing myself thoroughly, I have to ask: How do people  
> here
> handle dynamic default values?  Do leave these instructions in the
> controllers or is there an accepted technique for moving this code  
> into
> the model?  If there is then what is it? I ran across this in my
> research:
>
> http://m.onkey.org/2007/7/24/how-to-set-default-values-in-your-model
>
> But this is not what one would describe as self documenting code.

You could try this plugin:

http://github.com/aussiegeek/active_record_defaults/tree/master

Scott

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