Rails - Aug 2012 - Nested form with has_many :through -- how to modify the "through" attributes

This question occurred to me as I was answering a different question. Given a
has_many :through relationship, where there are additional attributes in the
:through model, how would you construct a form so you could update those
attributes?

#foo.rb
class Foo < ActiveRecord::Base
  has_many :bars
  has_many :bazzes, :through => :bars
  accepts_nested_attributes_for :bazzes
end

#bar.rb
class Bar < ActiveRecord::Base
  belongs_to :foo
  belongs_to :baz
  # also defines the ''blarg'' attribute
end

#baz.rb
class Baz < ActiveRecord::Base
  has_many :bars
  has_many :foos, :through => :bars
end

So in a form_for @foo, in fields_for :bazzes, how would I define a checkbox to
set the ''blarg'' attribute to true or false? Does the nested
@foo.baz just have a magical #blarg attribute, even though that attribute is
defined in the linking model Bar?

Thanks,

Walter

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On Aug 9, 2012, at 11:25 AM, Walter Lee Davis wrote:
> This question occurred to me as I was answering a different question. Given
a has_many :through relationship, where there are additional attributes in the
:through model, how would you construct a form so you could update those
attributes?
> 
> #foo.rb
> class Foo < ActiveRecord::Base
>  has_many :bars
>  has_many :bazzes, :through => :bars
>  accepts_nested_attributes_for :bazzes
> end
> 
> #bar.rb
> class Bar < ActiveRecord::Base
>  belongs_to :foo
>  belongs_to :baz
>  # also defines the ''blarg'' attribute
> end
> 
> #baz.rb
> class Baz < ActiveRecord::Base
>  has_many :bars
>  has_many :foos, :through => :bars
> end
> 
> So in a form_for @foo, in fields_for :bazzes, how would I define a checkbox
to set the ''blarg'' attribute to true or false? Does the nested
@foo.baz just have a magical #blarg attribute, even though that attribute is
defined in the linking model Bar?
I''ve gone so far as to scaffold this out, because I really would like
to know the answer! Aside from having the plural of Baz wrong, my code above
stands, and works as far as having multiple baz objects attached to a foo
through a bar, and being able to create and modify these in a nested form. This
part of Rails has only gotten easier to use in the latest version. What I cannot
seem to do is in any way change the Bar when saving the association of Baz from
a nested Foo form.

Here''s my view:

<%= form_for(@foo) do |f| %>
  <% if @foo.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@foo.errors.count, "error") %>
prohibited this foo from being saved:</h2>

      <ul>
      <% @foo.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

<!-- this all works swimmingly -->
  <%= f.fields_for :bazs do |b| %>
    <div class="field">
      <%= b.label :title, "Baz" %><br />
      <%= b.text_field :title %>

<!-- here be the error -->
      <%= b.check_box :blarged %> 
<!-- what do I do to set bar#blarged= in the context of h/m/t? -->

    </div>
  <% end %>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

Trying this in console, I can get the list of bazs associated with a foo, and if
I inspect one of those, I can see that it has a foo, but I can''t seem
to do anything to get the bar that stands between them and get or set any of its
properties.

Loading development environment (Rails 3.2.7)
1.9.2p320 :001 > f = Foo.last
  Foo Load (0.1ms)  SELECT "foos".* FROM "foos" ORDER BY
"foos"."id" DESC LIMIT 1
 => #<Foo id: 3, name: "I''m a Foo", created_at:
"2012-08-09 16:37:38", updated_at: "2012-08-09 16:37:38">
1.9.2p320 :002 > f.bars
  Bar Load (0.3ms)  SELECT "bars".* FROM "bars" WHERE
"bars"."foo_id" = 3
 => [#<Bar id: 1, blarged: nil, foo_id: 3, baz_id: 1, created_at:
"2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">, #<Bar id: 2, blarged: nil, foo_id: 3, baz_id: 2,
created_at: "2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">]
1.9.2p320 :004 > f.bazs
  Baz Load (0.2ms)  SELECT "bazs".* FROM "bazs" INNER JOIN
"bars" ON "bazs"."id" =
"bars"."baz_id" WHERE "bars"."foo_id" =
3
 => [#<Baz id: 1, title: "Here''s a baz", created_at:
"2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">, #<Baz id: 2, title: "Here''s another",
created_at: "2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">]
1.9.2p320 :005 > f.bazs.first.bar
NoMethodError: undefined method `bar'' for #<Baz:0x007ff89a9ba7b8>
	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/activemodel-3.2.7/lib/active_model/attribute_methods.rb:407:in
`method_missing''
	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/activerecord-3.2.7/lib/active_record/attribute_methods.rb:149:in
`method_missing''
	from (irb):5
	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands/console.rb:47:in
`start''
	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands/console.rb:8:in
`start''
	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:41:in
`<top (required)>''
	from script/rails:6:in `require''
	from script/rails:6:in `<main>''
1.9.2p320 :006 > 

It seems to me as though if I use the magical h/m/t  and nested form behavior, I
don''t get access to the specific bar being used to bridge one foo to
one baz. If I manually inspect a foo''s bars, I can see the extra
attribute (but I would have to rewrite my form and controller and not use the
magic). What''s the trick I need?

Thanks in advance,

Walter

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SOLVED: Note to future self:

On Aug 9, 2012, at 1:39 PM, Walter Lee Davis wrote:
> 
> On Aug 9, 2012, at 11:25 AM, Walter Lee Davis wrote:
> 
>> This question occurred to me as I was answering a different question.
Given a has_many :through relationship, where there are additional attributes in
the :through model, how would you construct a form so you could update those
attributes?
>> 
>> #foo.rb
>> class Foo < ActiveRecord::Base
>> has_many :bars
>> has_many :bazzes, :through => :bars
>> accepts_nested_attributes_for :bazzes
>> end
class Foo < ActiveRecord::Base
  attr_accessible :name, :bars_attributes
  has_many :bars
  has_many :bazs, :through => :bars, :dependent => :destroy
  accepts_nested_attributes_for :bars
end

>> 
>> #bar.rb
>> class Bar < ActiveRecord::Base
>> belongs_to :foo
>> belongs_to :baz
>> # also defines the ''blarg'' attribute
>> end
>> 
class Bar < ActiveRecord::Base
  attr_accessible :baz_id, :blarged, :foo_id, :baz_attributes, :baz
  belongs_to :baz
  belongs_to :foo
  accepts_nested_attributes_for :baz
end
>> #baz.rb
>> class Baz < ActiveRecord::Base
>> has_many :bars
>> has_many :foos, :through => :bars
>> end
class Baz < ActiveRecord::Base
  attr_accessible :title
  has_many :bars
  has_many :foos, :through => :bars, :dependent => :destroy
end
>> 
>> So in a form_for @foo, in fields_for :bazzes, how would I define a
checkbox to set the ''blarg'' attribute to true or false? Does
the nested @foo.baz just have a magical #blarg attribute, even though that
attribute is defined in the linking model Bar?
> 
> I''ve gone so far as to scaffold this out, because I really would
like to know the answer! Aside from having the plural of Baz wrong, my code
above stands, and works as far as having multiple baz objects attached to a foo
through a bar, and being able to create and modify these in a nested form. This
part of Rails has only gotten easier to use in the latest version. What I cannot
seem to do is in any way change the Bar when saving the association of Baz from
a nested Foo form.
> 
> Here''s my view:
> 
> <%= form_for(@foo) do |f| %>
>  <% if @foo.errors.any? %>
>    <div id="error_explanation">
>      <h2><%= pluralize(@foo.errors.count, "error") %>
prohibited this foo from being saved:</h2>
> 
>      <ul>
>      <% @foo.errors.full_messages.each do |msg| %>
>        <li><%= msg %></li>
>      <% end %>
>      </ul>
>    </div>
>  <% end %>
> 
>  <div class="field">
>    <%= f.label :name %><br />
>    <%= f.text_field :name %>
>  </div>
> 
> <!-- this all works swimmingly -->
>  <%= f.fields_for :bazs do |b| %>
>    <div class="field">
>      <%= b.label :title, "Baz" %><br />
>      <%= b.text_field :title %>
> 
> <!-- here be the error -->
>      <%= b.check_box :blarged %> 
> <!-- what do I do to set bar#blarged= in the context of h/m/t? -->
> 
>    </div>
>  <% end %>
>  <div class="actions">
>    <%= f.submit %>
>  </div>
> <% end %>
<%= form_for(@foo) do |f| %>
<% if @foo.errors.any? %>
<div id="error_explanation">
  <h2><%= pluralize(@foo.errors.count, "error") %>
prohibited this foo from being saved:</h2>

  <ul>
    <% @foo.errors.full_messages.each do |msg| %>
    <li><%= msg %></li>
    <% end %>
  </ul>
</div>
<% end %>

<div class="field">
  <%= f.label :name %><br />
  <%= f.text_field :name %>
</div>
<%= f.fields_for :bars do |b| %>
<div class="field">
  <%= b.fields_for :baz do |z| %>
  <%= z.label :title, "Baz" %><br />
  <%= z.text_field :title %>
  <% end %>
  <%= b.check_box :blarged %>
</div>
<% end %>
<div class="actions">
  <%= f.submit %>
</div>
<% end %>

> 
> Trying this in console, I can get the list of bazs associated with a foo,
and if I inspect one of those, I can see that it has a foo, but I can''t
seem to do anything to get the bar that stands between them and get or set any
of its properties.
> 
> Loading development environment (Rails 3.2.7)
> 1.9.2p320 :001 > f = Foo.last
>  Foo Load (0.1ms)  SELECT "foos".* FROM "foos" ORDER BY
"foos"."id" DESC LIMIT 1
> => #<Foo id: 3, name: "I''m a Foo", created_at:
"2012-08-09 16:37:38", updated_at: "2012-08-09 16:37:38">
> 1.9.2p320 :002 > f.bars
>  Bar Load (0.3ms)  SELECT "bars".* FROM "bars" WHERE
"bars"."foo_id" = 3
> => [#<Bar id: 1, blarged: nil, foo_id: 3, baz_id: 1, created_at:
"2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">, #<Bar id: 2, blarged: nil, foo_id: 3, baz_id: 2,
created_at: "2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">]
> 1.9.2p320 :004 > f.bazs
>  Baz Load (0.2ms)  SELECT "bazs".* FROM "bazs" INNER
JOIN "bars" ON "bazs"."id" =
"bars"."baz_id" WHERE "bars"."foo_id" =
3
> => [#<Baz id: 1, title: "Here''s a baz",
created_at: "2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">, #<Baz id: 2, title: "Here''s another",
created_at: "2012-08-09 16:37:38", updated_at: "2012-08-09
16:37:38">]
> 1.9.2p320 :005 > f.bazs.first.bar
> NoMethodError: undefined method `bar'' for
#<Baz:0x007ff89a9ba7b8>
> 	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/activemodel-3.2.7/lib/active_model/attribute_methods.rb:407:in
`method_missing''
> 	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/activerecord-3.2.7/lib/active_record/attribute_methods.rb:149:in
`method_missing''
> 	from (irb):5
> 	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands/console.rb:47:in
`start''
> 	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands/console.rb:8:in
`start''
> 	from
/Users/waltd/.rvm/gems/ruby-1.9.2-p320/gems/railties-3.2.7/lib/rails/commands.rb:41:in
`<top (required)>''
> 	from script/rails:6:in `require''
> 	from script/rails:6:in `<main>''
> 1.9.2p320 :006 > 
> 
> It seems to me as though if I use the magical h/m/t  and nested form
behavior, I don''t get access to the specific bar being used to bridge
one foo to one baz. If I manually inspect a foo''s bars, I can see the
extra attribute (but I would have to rewrite my form and controller and not use
the magic). What''s the trick I need?
The trick was to not try to build a bridge in mid-air. The foo could find a baz,
but that found baz did not seem to have any notion of which bar was standing
between the two. Nesting my forms one layer at a time did the trick. fields_for
bars then contained fields_for baz.

Walter
> 
> Thanks in advance,
> 
> Walter
> 
> -- 
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> 
> 
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Walter Davis wrote in post #1071859:
> SOLVED: Note to future self:
>
>
> The trick was to not try to build a bridge in mid-air. The foo could
> find a baz, but that found baz did not seem to have any notion of which
> bar was standing between the two. Nesting my forms one layer at a time
> did the trick. fields_for bars then contained fields_for baz.
>
> Walter
Thank you very much i appreciate it!!!

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