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

Editing applicant

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

<% if @applicant.skills.length > 0 %>

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 %>

<% end %>
<% end %>
Add skill? <%= text_field :newskill, :description, :size=>20 %>

<%= 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

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.

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 W. 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.

Stephan W. 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.

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 W. wrote:

Stephan W. 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.

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