Rails core - May 2012 - Allow to use Proc on :order option of associations

Take a look (not my production code but reproduces the desired behavior):

``` ruby
class Post < ActiveRecord::Base
  attr_accessible :published_at, :name

  has_and_belongs_to_many :categories, :order => lambda { 
[Category.arel_table[:published_at].desc, Category.arel_table[:name]] }
end
```

``` ruby
class Category < ActiveRecord::Base
  attr_accessible :published_at, :name

  has_and_belongs_to_many :posts, :order => lambda { 
[Post.arel_table[:published_at].desc, Post.arel_table[:name]] }
end
```

``` ruby
Category.first.posts
# => TypeError: Cannot visit Proc
```

``` ruby
Post.first.categories
# => TypeError: Cannot visit Proc
```

The problem happens when we have a cross reference between two models and 
need to use arel due to difference on quote of columns.

Sounds reasonable?

Reference: https://github.com/rails/rails/issues/6146

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There has been discussion about putting a new syntax into Rails 4 that 
more closely resembles scopes. Example:

has_many :comments, -> { where(deleted: false).order(:created_at) }

This would solve the problem of eager evaluation because it''s contained
in a lambda.

I''ll be looking into this when I get around to it.

On 03/05/12 19:28, Gabriel Sobrinho wrote:
> Take a look (not my production code but reproduces the desired behavior):
>
> ``` ruby
> class Post < ActiveRecord::Base
> attr_accessible :published_at, :name
>
> has_and_belongs_to_many :categories, :order => lambda {
> [Category.arel_table[:published_at].desc, Category.arel_table[:name]] }
> end
> ```
>
> ``` ruby
> class Category < ActiveRecord::Base
> attr_accessible :published_at, :name
>
> has_and_belongs_to_many :posts, :order => lambda {
> [Post.arel_table[:published_at].desc, Post.arel_table[:name]] }
> end
> ```
>
> ``` ruby
> Category.first.posts
> # => TypeError: Cannot visit Proc
> ```
>
> ``` ruby
> Post.first.categories
> # => TypeError: Cannot visit Proc
> ```
>
> The problem happens when we have a cross reference between two models
> and need to use arel due to difference on quote of columns.
>
> Sounds reasonable?
>
> Reference: https://github.com/rails/rails/issues/6146
>
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Thanks John!

On Friday, May 4, 2012 7:48:34 AM UTC-3, Jon Leighton
wrote:
>
> There has been discussion about putting a new syntax into Rails 4 that 
> more closely resembles scopes. Example: 
>
> has_many :comments, -> { where(deleted: false).order(:created_at) } 
>
> This would solve the problem of eager evaluation because it''s
contained
> in a lambda. 
>
> I''ll be looking into this when I get around to it. 
>
> On 03/05/12 19:28, Gabriel Sobrinho wrote: 
> > Take a look (not my production code but reproduces the desired 
> behavior): 
> > 
> > ``` ruby 
> > class Post < ActiveRecord::Base 
> > attr_accessible :published_at, :name 
> > 
> > has_and_belongs_to_many :categories, :order => lambda { 
> > [Category.arel_table[:published_at].desc, Category.arel_table[:name]]
}
> > end 
> > ``` 
> > 
> > ``` ruby 
> > class Category < ActiveRecord::Base 
> > attr_accessible :published_at, :name 
> > 
> > has_and_belongs_to_many :posts, :order => lambda { 
> > [Post.arel_table[:published_at].desc, Post.arel_table[:name]] } 
> > end 
> > ``` 
> > 
> > ``` ruby 
> > Category.first.posts 
> > # => TypeError: Cannot visit Proc 
> > ``` 
> > 
> > ``` ruby 
> > Post.first.categories 
> > # => TypeError: Cannot visit Proc 
> > ``` 
> > 
> > The problem happens when we have a cross reference between two models 
> > and need to use arel due to difference on quote of columns. 
> > 
> > Sounds reasonable? 
> > 
> > Reference: https://github.com/rails/rails/issues/6146 
> > 
> > -- 
> > You received this message because you are subscribed to the Google 
> > Groups "Ruby on Rails: Core" group. 
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> > https://groups.google.com/d/msg/rubyonrails-core/-/I5NDxR9PCvgJ. 
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rubyonrails-core@googlegroups.com.
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>
> -- 
> http://jonathanleighton.com/ 
>
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Em 04-05-2012 07:48, Jon Leighton escreveu:
> There has been discussion about putting a new syntax into Rails 4 that 
> more closely resembles scopes. Example:
>
> has_many :comments, -> { where(deleted: false).order(:created_at) }
For this particular case, I don''t understand. What would be the 
advantage of lazy evaluation?

Take a look at how some examples are handled in Sequel with regards to 
associations:

http://sequel.rubyforge.org/rdoc/files/doc/association_basics_rdoc.html

See section Association Options

So, for that particular case, a single block would be enough without the 
need for a proc. Here is an example using Sequel:

many_to_one(:comments){|ds| ds.where(deleted: false).order(:created_at) }

Procs would be useful for some cases where you depend on some dynamic 
data (taken from the Sequel documentation with a small typo fix):

Artist.one_to_many :songs, dataset: ->  {
   Song.select(:songs.*).join(Lyric, id: :lyric_id,
     id =>  [:composer_id, :arranger_id, :vocalist_id, :lyricist_id])
}
Artist.first.songs_dataset
# SELECT songs.* FROM songs
# INNER JOIN lyrics ON
#  lyrics.id = songs.lyric_id AND
#  1 IN (composer_id, arranger_id, vocalist_id, lyricist_id)


Also, I much prefer such approach as a generalized one than the AR 
''finder_sql'' option in a sense that it could be more easily
cascaded
with other conditions.

Am I missing something?

Cheers,
Rodrigo.

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Yep, cross-reference.

Both file needs to require the other to load :)


Category definition needs Post defined
Post definition needs Category defined


We simply can''t load these files in this way.

On May 4, 2012, at 10:54 AM, Rodrigo Rosenfeld Rosas wrote:
> Em 04-05-2012 07:48, Jon Leighton escreveu:
>> 
>> There has been discussion about putting a new syntax into Rails 4 that
more closely resembles scopes. Example:
>> 
>> has_many :comments, -> { where(deleted: false).order(:created_at) } 
> 
> For this particular case, I don''t understand. What would be the
advantage of lazy evaluation?
> 
> Take a look at how some examples are handled in Sequel with regards to
associations:
> 
> http://sequel.rubyforge.org/rdoc/files/doc/association_basics_rdoc.html
> 
> See section Association Options
> 
> So, for that particular case, a single block would be enough without the
need for a proc. Here is an example using Sequel:
> 
> many_to_one(:comments){|ds| ds.where(deleted: false).order(:created_at) }
> 
> Procs would be useful for some cases where you depend on some dynamic data
(taken from the Sequel documentation with a small typo fix):
> 
> Artist.one_to_many :songs, dataset: -> {
>   Song.select(:songs.*).join(Lyric, id: :lyric_id,
>     id => [:composer_id, :arranger_id, :vocalist_id, :lyricist_id])
> }
> Artist.first.songs_dataset
> # SELECT songs.* FROM songs
> # INNER JOIN lyrics ON
> #  lyrics.id = songs.lyric_id AND
> #  1 IN (composer_id, arranger_id, vocalist_id, lyricist_id)
> 
> Also, I much prefer such approach as a generalized one than the AR
''finder_sql'' option in a sense that it could be more easily
cascaded with other conditions.
> 
> Am I missing something?
> 
> Cheers,
> Rodrigo.
> 
> 
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Cheers,

Gabriel Sobrinho

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Ah, I see, thanks.

Em 04-05-2012 11:14, Gabriel Sobrinho escreveu:
> Yep, cross-reference.
>
> Both file needs to require the other to load :)
>
>
> Category definition needs Post defined
> Post definition needs Category defined
>
>
> We simply can''t load these files in this way.
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