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Stephan Wehner

2006-Jul-02 06:35 UTC

[Rails] Question about setting field values for a belongs_to model

I have two tables, applicants, and skills where each applicant 
"has_many" skills, and each skill "belongs_to" an applicant.

When I have a reference to an applicant and a (skill) id for one of 
their skills, I thought to update their skills by using

 @applicant.skills.find(id_of_skill).description = ''new
description''
# even adding @applicant.skills.find(id_of_skill).save here doesn''t
help
 ...other processing
@applicant.save

This sounds to me like it should lead to the description of the skill 
being updated in the database, and indeed an update statement is in the 
log, but with the original value. The description of the skill is not 
updated in the database.

Instead, this works

 @applicant.skills.find(id_of_skill).update_attributes(:description => 
''new description'')
...other processing
@applicant.save

So somewhere my intuition is wrong. How should I think about this? Why 
does the straightforward statement not work?

Code to pour over is below. This is on ruby 1.8.4 (2005-12-24) 
[i386-linux] / Rails 1.1.2

Thanks,

Stephan

-------------------------------------------
Following are

1. mysql table definitions
2. method update in ApplicantController
3. rhtml file, app/views/applicant/edit.rhtml
4. Applicant and Skill models


create table if not exists applicants (
  id int not null auto_increment,
  name varchar(20) not null,
  primary key(id),
  unique index uniq_index(id)
);

create table if not exists skills (
  id int not null auto_increment,
  applicant_id int not null,
  description varchar(30) not null,
  primary key(id),
  unique index uniq_index(id)
);

class ApplicantController < ApplicationController
:
: methods generated by scaffold
:

  def update
    @applicant = Applicant.find(params[:id])
    @applicant.attributes=params[:applicant]

    if !params[:newskill][:description].blank?
      @applicant.skills << Skill.new(params[:newskill])
    end
    if params[:skill]
      params[:skill].each do |skillid, newdescription|
      @applicant.skills.find(skillid.to_i).update_attributes( 
:description=> newdescription )
      #
      # The following two lines do not work, but they don''t crash
either
      # The log file shows update statements to the skill table
      # but with the original values, not the new values
      #
      # @applicant.skills.find(skillid.to_i).description = newdescription
      # @applicant.skills.find(skillid.to_i).save
      end
    end
    if params[:deleteskill]
      params[:deleteskill].each do |id, todelete|
        if todelete == "1"
          @applicant.skills.find(id).destroy
        end
      end
    end
    if (@applicant.save)
      flash[:notice] = ''Applicant was successfully updated.''
      redirect_to :action => ''show'', :id => @applicant
    else
      render :action => ''edit''
    end
  end

:
: other ApplicantController methods defined by scaffold
:

File app/views/applicant/edit.rhtml
<h1>Editing applicant</h1>

<%= start_form_tag :action => ''update'', :id =>
@applicant %>
  <%= render :partial => ''form'' %>

  <% if @applicant.skills.length > 0 %>
    <P>Skills:
    <% for skill in @applicant.skills %>
       Delete? <%= check_box :deleteskill, skill.id %>
       Change? <%= text_field :skill, skill.id.to_s, :size=>20, 
:value=>skill.description %> <br>
    <% end %>
  <% end %>
  Add skill? <%= text_field :newskill, :description, :size=>20
%><br>
  <%= submit_tag ''Edit'' %>
<%= end_form_tag %>

<%= link_to ''Show'', :action => ''show'',
:id => @applicant %> |
<%= link_to ''Back'', :action => ''list''
%>

The two models:

class Applicant < ActiveRecord::Base
  has_many :skills, :dependent => :destroy

end
class Skill < ActiveRecord::Base
  belongs_to :applicant
end

Stephan Wehner

2006-Jul-02 06:36 UTC

head link

[Rails] Question about setting field values for a belongs_to model

I have two tables, applicants, and skills where each applicant 
"has_many" skills, and each skill "belongs_to" an applicant.

When I have a reference to an applicant and a (skill) id for one of 
their skills, I thought to update their skills by using

@applicant.skills.find(id_of_skill).description = ''new
description''
# even adding @applicant.skills.find(id_of_skill).save here doesn''t
help
...other processing
@applicant.save

This sounds to me like it should lead to the description of the skill 
being updated in the database, and indeed an update statement is in the 
log, but with the original value. The description of the skill is not 
updated in the database.

Instead, this works

@applicant.skills.find(id_of_skill).update_attributes(:description => 
''new description'')
...other processing
@applicant.save

So somewhere my intuition is wrong. How should I think about this? Why 
does the straightforward statement not work?

Code to pour over is below. This is on ruby 1.8.4 (2005-12-24) 
[i386-linux] / Rails 1.1.2

Thanks,

Stephan

-------------------------------------------
Following are

1. mysql table definitions
2. method update in ApplicantController
3. rhtml file, app/views/applicant/edit.rhtml
4. Applicant and Skill models


create table if not exists applicants (
 id int not null auto_increment,
 name varchar(20) not null,
 primary key(id),
 unique index uniq_index(id)
);

create table if not exists skills (
 id int not null auto_increment,
 applicant_id int not null,
 description varchar(30) not null,
 primary key(id),
 unique index uniq_index(id)
);

class ApplicantController < ApplicationController
:
: methods generated by scaffold
:

 def update
   @applicant = Applicant.find(params[:id])
   @applicant.attributes=params[:applicant]

   if !params[:newskill][:description].blank?
     @applicant.skills << Skill.new(params[:newskill])
   end
   if params[:skill]
     params[:skill].each do |skillid, newdescription|
     @applicant.skills.find(skillid.to_i).update_attributes( 
:description=> newdescription )
     #
     # The following two lines do not work, but they don''t crash either
     # The log file shows update statements to the skill table
     # but with the original values, not the new values
     #
     # @applicant.skills.find(skillid.to_i).description = newdescription
     # @applicant.skills.find(skillid.to_i).save
     end
   end
   if params[:deleteskill]
     params[:deleteskill].each do |id, todelete|
       if todelete == "1"
         @applicant.skills.find(id).destroy
       end
     end
   end
   if (@applicant.save)
     flash[:notice] = ''Applicant was successfully updated.''
     redirect_to :action => ''show'', :id => @applicant
   else
     render :action => ''edit''
   end
 end

:
: other ApplicantController methods defined by scaffold
:

File app/views/applicant/edit.rhtml
<h1>Editing applicant</h1>

<%= start_form_tag :action => ''update'', :id =>
@applicant %>
 <%= render :partial => ''form'' %>

 <% if @applicant.skills.length > 0 %>
   <P>Skills:
   <% for skill in @applicant.skills %>
      Delete? <%= check_box :deleteskill, skill.id %>
      Change? <%= text_field :skill, skill.id.to_s, :size=>20, 
:value=>skill.description %> <br>
   <% end %>
 <% end %>
 Add skill? <%= text_field :newskill, :description, :size=>20
%><br>
 <%= submit_tag ''Edit'' %>
<%= end_form_tag %>

<%= link_to ''Show'', :action => ''show'',
:id => @applicant %> |
<%= link_to ''Back'', :action => ''list''
%>

The two models:

class Applicant < ActiveRecord::Base
 has_many :skills, :dependent => :destroy

end
class Skill < ActiveRecord::Base
 belongs_to :applicant
end

Alex Wayne

2006-Jul-02 06:49 UTC

head link

[Rails] Re: Question about setting field values for a belongs_to mod

If you directly manipulate the associated object, the parent object will 
become stale since the data it tracks is recorded inside the @applicant 
object.

Try "@applicant.reload" after updating an associated record.

Also, why are you doing this?

  @applicant.skills.find(id_of_skill).update_attributes(...)

As long as you have the id of the skill why not call it on the Skill 
clas directly?

  Skill.find(id_of_skill).update_attributes(...)

If you do this before creating the @applicant object you wont need ot 
call reload at all.

-- 
Posted via http://www.ruby-forum.com/.

Stephan Wehner

2006-Jul-02 16:17 UTC

head link

[Rails] Re: Question about setting field values for a belongs_to mod

Ok, thanks a lot, Alex. Why does the database get updated
when using "update_attributes" ? As far as I can see the
source code is

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

which looks a lot like my

    @applicant.skills.find(skillid.to_i).description = newdescription
    @applicant.skills.find(skillid.to_i).save


Stephan

Alex Wayne wrote:> If you directly manipulate the associated object, the parent object will 
> become stale since the data it tracks is recorded inside the @applicant 
> object.
> 
> Try "@applicant.reload" after updating an associated record.
> 
> Also, why are you doing this?
> 
>   @applicant.skills.find(id_of_skill).update_attributes(...)
> 
> As long as you have the id of the skill why not call it on the Skill 
> clas directly?
> 
>   Skill.find(id_of_skill).update_attributes(...)
> 
> If you do this before creating the @applicant object you wont need ot 
> call reload at all.

-- 
Posted via http://www.ruby-forum.com/.

Alex Wayne

2006-Jul-02 16:49 UTC

head link

[Rails] Re: Question about setting field values for a belongs_to mod

Stephan Wehner wrote:> Ok, thanks a lot, Alex. Why does the database get updated
> when using "update_attributes" ? As far as I can see the
> source code is
> 
>       def update_attributes(attributes)
>         self.attributes = attributes
>         save
>       end
> 
> which looks a lot like my
> 
>     @applicant.skills.find(skillid.to_i).description = newdescription
>     @applicant.skills.find(skillid.to_i).save
> 
> 
> Stephan
The problem is that each ActiveRecord object caches it''s fetched 
associations to reduce database load.

  @applicant.skills #fetches form database
  @applicant.skills #same result but from the object''s cache

  skill = Skill.find(1)
  skill.foo = ''bar''
  skill.save

  @applicant.skills #same result as above because this object
                    #doesn''t know anything changed so doesnt
                    #talk to db

  @applicant.reload.skills #cache is cleared and reloaded giving
                           #us correct results.

Think of the internal working of association fetches like this:

  def skills
    @skills ||= find_by_sql("SELECT FROM `skills` WHERE `applicant_id` = 
?", id)
  end

This means it will only execute the query if @skills is nil.

There are operations which do work without reloading however.  Such as:

  @applicant.skills << Skill.new(:foo => ''bar'')

lastly> Why does the database get updated when using "update_attributes"?
Because the update_attributes method calls save.  But this saves the 
skill, not the applicant because you are invoking the method that does 
the saving on the skill, not on the applicant.

-- 
Posted via http://www.ruby-forum.com/.

Stephan Wehner

2006-Jul-02 18:28 UTC

head link

[Rails] Re: Question about setting field values for a belongs_to mod

Ok, I found that

  skill = @applicant.skills.find(skillid.to_i)
  skill.description = newdescription
  skill.save

does "work" (the database gets updated).

In this sequence

  @applicant.skills.find(skillid.to_i).description = newdescription
  @applicant.skills.find(skillid.to_i).save

the second call to

  @applicant.skills.find(skillid.to_i)

does not return the object that was manipulated in the first line.

Thanks again. This mailing list is great!

Stephan

Alex Wayne wrote:> Stephan Wehner wrote:
>> Ok, thanks a lot, Alex. Why does the database get updated
>> when using "update_attributes" ? As far as I can see the
>> source code is
>> 
>>       def update_attributes(attributes)
>>         self.attributes = attributes
>>         save
>>       end
>> 
>> which looks a lot like my
>> 
>>     @applicant.skills.find(skillid.to_i).description = newdescription
>>     @applicant.skills.find(skillid.to_i).save
>> 
>> 
>> Stephan
> 
> The problem is that each ActiveRecord object caches it''s fetched 
> associations to reduce database load.
> 
>   @applicant.skills #fetches form database
>   @applicant.skills #same result but from the object''s cache
> 
>   skill = Skill.find(1)
>   skill.foo = ''bar''
>   skill.save
> 
>   @applicant.skills #same result as above because this object
>                     #doesn''t know anything changed so doesnt
>                     #talk to db
> 
>   @applicant.reload.skills #cache is cleared and reloaded giving
>                            #us correct results.
> 
> Think of the internal working of association fetches like this:
> 
>   def skills
>     @skills ||= find_by_sql("SELECT FROM `skills` WHERE `applicant_id`
=
> ?", id)
>   end
> 
> This means it will only execute the query if @skills is nil.
> 
> There are operations which do work without reloading however.  Such as:
> 
>   @applicant.skills << Skill.new(:foo => ''bar'')
> 
> lastly
>> Why does the database get updated when using
"update_attributes"?
> 
> Because the update_attributes method calls save.  But this saves the 
> skill, not the applicant because you are invoking the method that does 
> the saving on the skill, not on the applicant.

-- 
Posted via http://www.ruby-forum.com/.

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