Rails - Sep 2005 - ActiveRecord Flaw Makes It Impossible To Do Previews?

Hi all.

I''ve just gone two days from startlement to lack of comprehension  
that it seems not to be possible to implement previews with Ruby On  
Rails without letting down the elegance my code usually has with Rails.

In my opinion there is a very major flaw in ActiveRecord that makes  
previews close to impossible: About every time you change something  
in Ruby, you automatically change the database. Mow. While this is no  
problem as long as I really want this, it really makes no sense for  
many application that require a "Preview" button.

Of course, you can set attributes using model.attributes= or set them  
bit by bit. However, the "favourite" method -
"update_attributes"
automatically changes the database state.

Additionally, when updating relations, these changes are *always*  
persisted immediately - but for when you change relations on a yet  
unsaved object.

Okay, I thought, just go ahead, create an unsaved clone of your model  
object, change whatever you want there and push that to the preview.  
Then, simply call model.valid? to force validation without saving.  
However, this does not work, of course, if you have an  
"validate_uniqueness_of" constraint on one of your model''s
fields.

I want to have a form to edit articles with a "preview" and a  
"update" button where the preview button only displays the form again
and then a preview of my article. My article model has a n:m relation  
to the author model. Article titles are unique.

So I seem to be stuck with the following options:

  * Set the non-relation attributes of the primary class and call  
model.valid? to validate the simple attributes. Then create a clone,  
change its attributes and set it into the template for preview. This  
works for my case but when you want to validate attributes too you  
have  to create a clone with fake attributes so you don''t break the  
unique constraints. Well, this is very hacky!

   * Set the attributes of the model manually, then call validate on  
them. Put the relation attributes to the template into differen  
variables. This is not very elegant.

Why can''t I simply do something like the following?

   @article.find params[:id]
   @article.title = params[:article][:title]
   ...
   params[:article][:authors].each do |author_id|
     @article.authors << Author.find author_id
   end

Is there something obvious I am missing? Has anyone implemented a  
preview feature? Or is it just that nobody does previews? No Rails  
based open source application supports previews the way I described  
above. Even Basecamp has no previews.

I would really appreciate any hint into the right direction.

Regards

Manuel Holtgrewe

Manuel Holtgrewe <purestorm-nlpEiS6K5uusTnJN9+BGXg@public.gmane.org>
writes:
> Of course, you can set attributes using model.attributes= or set them  
> bit by bit. However, the "favourite" method -
"update_attributes"
> automatically changes the database state.
Yes, I''ve been frustrated by this before too.
> Is there something obvious I am missing? Has anyone implemented a  
> preview feature? Or is it just that nobody does previews? No Rails  
> based open source application supports previews the way I described  
> above. Even Basecamp has no previews.
I''ve been playing with acts_as_versioned this morning.  Not too tricky
to setup.  It took me two hours, but most of that was me just being
stupid and getting distracted.

What this would let you to is go ahead and save your object with
update_attributes, and then revert_to the previous version if they
cancel after preview.

http://wiki.rubyonrails.com/rails/show/ActsAsVersioned
-- 
Doug Alcorn - http://lathi.net/RubyOnRailsDeveloper
doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org

In article
<m2ek7o3jkj.fsf-HEBgovpM+ff0Kn81hEinErNAH6kLmebB@public.gmane.org>, doug-
jGAhs73c5XxeoWH0uzbU5w-XMD5yJDbdMReXY1tMh2IBg@public.gmane.org
says...
> It took me two hours, but most of that was me just being
> stupid and getting distracted.
Doug:

If you find a workaround for that, please let me know - I''ve been
having
the same problems with Rails.

-- 
Jay Levitt                | 
Wellesley, MA             | I feel calm.  I feel ready.  I can only
Faster: jay at jay dot fm | conclude that''s because I don''t
have a
http://www.jay.fm         | full grasp of the situation. - Mark Adler

Am 16.09.2005 um 21:21 schrieb Doug Alcorn:
> I''ve been playing with acts_as_versioned this morning.  Not too
tricky
> to setup.  It took me two hours, but most of that was me just being
> stupid and getting distracted.
>
> What this would let you to is go ahead and save your object with
> update_attributes, and then revert_to the previous version if they
> cancel after preview.
Well, when they simply close their browser window you are stuck with  
the changes - or am I mistaken there?

One thing I could imagine is starting a session, change the object  
and then cancel the transaction by throwing an Exception (gah!).  
Another possibility would be doing a "deep clone" using marshall/ 
unmarshall and then undoing things manually instead of throwing the  
Exception.

This offers possible solutions to the problems. However, these  
solutions are neither beautiful nor elegant nor will they offer good  
performance.

I guess if I do not find a good way to do this, I will override  
"validates_uniqueness_of" of ActiveRecord so even if the record has  
not been saved yet, it ignores duplicates with the same id. The  
"validates_uniqueness_of" method currently checks for the record to  
be changed being saved yet. If it is not, it excludes the record from  
its "SELECT * FROM record WHERE property = #{value}" query with
"AND
id <> #{current_id}".

If anyone has a more elegant and consistent way of implementing  
previews, I''d desperately like to read it!

Regards

Manuel Holtgrewe

Manuel Holtgrewe <purestorm-nlpEiS6K5uusTnJN9+BGXg@public.gmane.org>
writes:
> One thing I could imagine is starting a session, change the object  
> and then cancel the transaction by throwing an Exception (gah!).  
> Another possibility would be doing a "deep clone" using marshall/
> unmarshall and then undoing things manually instead of throwing the  
> Exception.
It wouldn''t be that hard to re-implement update_attributes so that it
doesn''t save.

 def update_attributes_no_save(hash)
   hash.keys.each do |key|
      if self.responds_to?("#{key}=")
         self.send("#{key}=", hash[:key])
      end
   end
 end

I think that''s basically what update_attributes does and then does a
self.save.
-- 
Doug Alcorn - http://lathi.net/RubyOnRailsDeveloper
doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org

* Doug Alcorn (doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org) [050916
15:10]:
> It wouldn''t be that hard to re-implement update_attributes so that
it
> doesn''t save.
> 
>  def update_attributes_no_save(hash)
>    hash.keys.each do |key|
>       if self.responds_to?("#{key}=")
>          self.send("#{key}=", hash[:key])
>       end
>    end
>  end
> 
> I think that''s basically what update_attributes does and then does
a
> self.save.
Seems like the simplest refactoring then would be to take the non-save
part of update_attributes(), extract it as something like
update_attributes_no_save, and then change update_attributes() to be:

def update_attributes(hash)
  update_attributes_no_save(hash)
  self.save()
end

Rick
-- 
 http://www.rickbradley.com    MUPRN: 523
                       |  I am searching
   random email haiku  |  and then go away and read
                       |  them somewhere else.

> Seems like the simplest refactoring then would be to take the non-save
> part of update_attributes(), extract it as something like
> update_attributes_no_save, and then change update_attributes() to be:
> 
> def update_attributes(hash)
>   update_attributes_no_save(hash)
>   self.save()
> end
Look at the actual source of update_attributes:

def update_attributes(attributes)
  self.attributes = attributes
  save
end

That should tell you what you need :)

Doug: I added a tip about creating the versioned table to that wiki
page.  I''ll work on some decent docs this weekend for the thing.  Hope
it works out for you..

-- 
rick
http://techno-weenie.net

Rick Olson <technoweenie-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> Doug: I added a tip about creating the versioned table to that wiki
> page.  I''ll work on some decent docs this weekend for the thing.
> Hope it works out for you..
I saw that create_versioned_table method, but wasn''t sure how to use
it.  Your migration example is perfect, thanks!
-- 
Doug Alcorn - http://lathi.net/RubyOnRailsDeveloper
doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org

Am 16.09.2005 um 22:55 schrieb Rick Olson:
> Look at the actual source of update_attributes:
>
> def update_attributes(attributes)
>   self.attributes = attributes
>   save
> end
>
> That should tell you what you need :)
Yes, as I wrote:
>>> Of course, you can set attributes using model.attributes= or set  
>>> them bit by bit. However, the "favourite" method -  
>>> "update_attributes" automatically changes the database
state.
But this does not help with relations. Relations are stored to the  
database immediately. They are not stored to the database for records  
not already in the database.

When I create a new record not in the database, assign attributes to  
it and call model.valid?, I get errors because the uniqueness  
validator method rightfully detects that I have a new record that has  
the same title as an already existing record. It would be no  
workaround to remove uniqueness validation depending on a  
"is_preview" flag since I need uniqueness validation - but it should  
ignore just one other record in the database: The that has the same  
id as the current model object.

Do you have any idea of how to implement setting relations without  
saving with ActiveRecord? Model.collection.build() is not a solution  
since it is "add only" and seems only to work for new objects.

Regards

Manuel Holtgrewe

Jay Levitt <jay-news-WxwZQdyI2t0@public.gmane.org> writes:
> In article
<m2ek7o3jkj.fsf-HEBgovpM+ff0Kn81hEinErNAH6kLmebB@public.gmane.org>, doug-
> jGAhs73c5XxeoWH0uzbU5w-XMD5yJDbdMReXY1tMh2IBg@public.gmane.org says...
>> It took me two hours, but most of that was me just being
>> stupid and getting distracted.
>
> Doug:
>
> If you find a workaround for that, please let me know - I''ve been
having
> the same problems with Rails.
Ha!  The best antidote for both those problems is pair programming.
-- 
Doug Alcorn - http://lathi.net/RubyOnRailsDeveloper
doug-jGAhs73c5XxeoWH0uzbU5w@public.gmane.org