Rails core - Jul 2012 - Adding new ARel visitors to resolve TypeError: Cannot visit ClassName

Is there an approved way to add or register a new ARel visitor without 
adding it to the ARel source? Looking at the some of the source
code<https://github.com/rails/arel/blob/master/lib/arel/visitors/to_sql.rb#L434-466>
it
appears that new visitors are just hard coded as a new methods or aliases.

The reason I''m asking is because I get  "TypeError: Cannot visit
ClassName"
when I override reader methods with some sort of composed object (similar 
to Active Record''s composed_of) and use that attribute in a uniqueness 
validation. 

Take the following for example:

class Cat < ActiveRecord::Base
  validates :name, :uniqueness => { :scope => [:breed] }

  def breed
    CatBreed.new(super)
  end
end

class CatBreed
  def initialize(breed)
    @breed = breed
  end

  def to_s
    @breed
  end
end

The above code will raise "TypeError: Cannot visit CatBreed" when I
try to
create a new cat unless I add a visitor method to Arel::Visitors::ToSql 
called visit_CatBreed:

class Arel::Visitors::ToSql
  def visit_CatBreed(value)
    quoted(value.to_s)
  end
end

I''m hoping there is a better way to deal with this problem, either with
a
less hacky ARel approach or something different all together.


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On Sun, Jul 15, 2012 at 07:01:38AM -0700, Peter Brown
wrote:
> Is there an approved way to add or register a new ARel visitor without 
> adding it to the ARel source? Looking at the some of the source
code<https://github.com/rails/arel/blob/master/lib/arel/visitors/to_sql.rb#L434-466>
it
> appears that new visitors are just hard coded as a new methods or aliases.
> 
> The reason I''m asking is because I get  "TypeError: Cannot
visit ClassName"
> when I override reader methods with some sort of composed object (similar 
> to Active Record''s composed_of) and use that attribute in a
uniqueness
> validation. 
> 
> Take the following for example:
> 
> class Cat < ActiveRecord::Base
>   validates :name, :uniqueness => { :scope => [:breed] }
> 
>   def breed
>     CatBreed.new(super)
>   end
> end
> 
> class CatBreed
>   def initialize(breed)
>     @breed = breed
>   end
> 
>   def to_s
>     @breed
>   end
> end
> 
> The above code will raise "TypeError: Cannot visit CatBreed" when
I try to
> create a new cat unless I add a visitor method to Arel::Visitors::ToSql 
> called visit_CatBreed:
> 
> class Arel::Visitors::ToSql
>   def visit_CatBreed(value)
>     quoted(value.to_s)
>   end
> end
Doing this is fine for now.  The way the AST is translated to SQL won''t
change, so this will probably not break in the future.
> I''m hoping there is a better way to deal with this problem, either
with a
> less hacky ARel approach or something different all together.
It seems like we should be using the underlying database value (the
attribute value rather than the return value of the method call) for the
unique constraint.  I think this might be a bug.  Jon, wdyt?

-- 
Aaron Patterson
http://tenderlovemaking.com/

Aaron, 

Thanks for the reply. I played around with it a little and it seems like 
changing how the scope_value is retrieved in 
ActiveRecordmodule::Validationsclass::UniquenessValidator<https://github.com/rails/rails/blob/master/activerecord/lib/active_record/validations/uniqueness.rb#L29-32>would
fix the issue (though I''m not sure what sort of repercussions this
would have elsewhere)

scope_value = record.send(scope_item)


to either of these

record[scope_item]
record.read_attribute(scope_item)



On Sunday, July 15, 2012 1:50:01 PM UTC-4, Aaron Patterson
wrote:
>
> On Sun, Jul 15, 2012 at 07:01:38AM -0700, Peter Brown wrote: 
> > Is there an approved way to add or register a new ARel visitor without
> > adding it to the ARel source? Looking at the some of the source
code<
>
https://github.com/rails/arel/blob/master/lib/arel/visitors/to_sql.rb#L434-466>
> it 
> > appears that new visitors are just hard coded as a new methods or 
> aliases. 
> > 
> > The reason I''m asking is because I get  "TypeError:
Cannot visit
> ClassName" 
> > when I override reader methods with some sort of composed object 
> (similar 
> > to Active Record''s composed_of) and use that attribute in a
uniqueness
> > validation. 
> > 
> > Take the following for example: 
> > 
> > class Cat < ActiveRecord::Base 
> >   validates :name, :uniqueness => { :scope => [:breed] } 
> > 
> >   def breed 
> >     CatBreed.new(super) 
> >   end 
> > end 
> > 
> > class CatBreed 
> >   def initialize(breed) 
> >     @breed = breed 
> >   end 
> > 
> >   def to_s 
> >     @breed 
> >   end 
> > end 
> > 
> > The above code will raise "TypeError: Cannot visit CatBreed"
when I try
> to 
> > create a new cat unless I add a visitor method to
Arel::Visitors::ToSql
> > called visit_CatBreed: 
> > 
> > class Arel::Visitors::ToSql 
> >   def visit_CatBreed(value) 
> >     quoted(value.to_s) 
> >   end 
> > end 
>
> Doing this is fine for now.  The way the AST is translated to SQL
won''t
> change, so this will probably not break in the future. 
>
> > I''m hoping there is a better way to deal with this problem,
either with
> a 
> > less hacky ARel approach or something different all together. 
>
> It seems like we should be using the underlying database value (the 
> attribute value rather than the return value of the method call) for the 
> unique constraint.  I think this might be a bug.  Jon, wdyt? 
>
> -- 
> Aaron Patterson 
> http://tenderlovemaking.com/ 
>
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>> I''m hoping there is a better way to deal with this problem,
either with a
>> less hacky ARel approach or something different all together.
>
> It seems like we should be using the underlying database value (the
> attribute value rather than the return value of the method call) for the
> unique constraint.  I think this might be a bug.  Jon, wdyt?
Yes, I agree. Who wants to create a patch? :)

-- 
http://jonathanleighton.com/


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Me :) https://github.com/rails/rails/pull/7072

On Monday, July 16, 2012 3:36:39 PM UTC-4, Jon Leighton
wrote:
>
> >> I''m hoping there is a better way to deal with this
problem, either with
> a 
> >> less hacky ARel approach or something different all together. 
> > 
> > It seems like we should be using the underlying database value (the 
> > attribute value rather than the return value of the method call) for
the
> > unique constraint.  I think this might be a bug.  Jon, wdyt? 
>
> Yes, I agree. Who wants to create a patch? :) 
>
> -- 
> http://jonathanleighton.com/ 
>
>
>
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