Rails 3 - dependent => :delete_all doesn't work. Why? - ruby-on-rails

Question

I have few models in my project. There are some of them: Qualification and Curriculum. Qualification has children (curriculums). I want to make sure that when i delete Qualification i delete all it's children. Here is me code:
# Table name: qualifications
#
# id :integer not null, primary key
# subject_id :integer
# teacher_id :integer
# created_at :datetime not null
# updated_at :datetime not null
class Qualification < ActiveRecord::Base
belongs_to :subject
belongs_to :teacher
has_many :curriculums, :dependent => :delete_all
has_many :school_classes, :through => :curriculums
end
# id :integer not null, primary key
# school_class_id :integer
# qualification_id :integer
# created_at :datetime not null
# updated_at :datetime not null
class Curriculum < ActiveRecord::Base
belongs_to :school_class
belongs_to :qualification
has_one :result
has_many :timetables
end
As you can see i tried to use :dependent => :delete_all in Qualification model. But it doesn't work. Why?
UPD:
I deleting qualification by unchecking checkboxes in view when i EDIT it:
<div class="control-group">
<%= f.label :subject_ids, "Teacher can teach such subjects in the school",
:class => "control-label" %>
<div class="controls">
<table class="table table-bordered table-striped">
<thead>
<tr>
<th>Choose</th>
<th>Subject</th>
</tr>
</thead>
<tbody>
<%= hidden_field_tag "teacher[subject_ids][]", nil %> <%# We use hidden field because it doesn't submit unchecked fields. So, we pass nil and nothing will be submitted.%>
<% #subjects.each do |subject| %>
<tr>
<td>
<%= check_box_tag "teacher[subject_ids][]", # [] brackets tells that this is array.
subject.id, # Value of checkbox.
#teacher.subject_ids.include?(subject.id), # Here we automatically check checkboxes.
id: dom_id( subject ) %> <%# Give unique id for each value. 'dom_id' is Rails helper. We will have ids like: 'subject_1', 'subject_2' and etc. %>
</td>
<td>
<%= label_tag dom_id( subject ), subject.subject_name %> <%# Put name of subject. %>
</td>
</tr>
<% end %>
</tbody>
</table>
</div>
</div>
Here is more info:
class Teacher < ActiveRecord::Base
has_many :qualifications
has_many :subjects, :through => :qualifications
end
class Subject < ActiveRecord::Base
has_many :qualifications
has_many :teachers, :through => :qualifications
end
Here is SQL code when i update my model:
Started PUT "/teachers/2" for 127.0.0.1 at 2012-06-03 18:34:44 +0400
Processing by TeachersController#update as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"ZJNNV9/TO6k18O1Ar1kpkU+PWbd7btHm9Tc067iMNO4=", "teacher"=>{"teacher_last_name"=>"Last", "teacher_first_name"=>"First", "teacher_middle_name"=>"Middle", "teacher_sex"=>"m", "teacher_birthday"=>"1980-12-01", "teacher_phone_attributes"=>{"teacher_mobile_number"=>"88283686", "teacher_home_number"=>"5112787", "id"=>"2"}, "teacher_education_attributes"=>{"teacher_education_university"=>"Mmm", "teacher_education_year"=>"1970-01-01", "teacher_education_graduation"=>"Graduated", "teacher_education_speciality"=>"Math", "id"=>"2"}, "teacher_category"=>"1st", "subject_ids"=>["", "4", "3", "1"]}, "commit"=>"Update", "id"=>"2"}
User Load (0.5ms) SELECT "users".* FROM "users" WHERE "users"."id" = 2 LIMIT 1
Teacher Load (0.4ms) SELECT "teachers".* FROM "teachers" WHERE "teachers"."id" = $1 LIMIT 1 [["id", "2"]]
(0.1ms) BEGIN
Subject Load (0.5ms) SELECT "subjects".* FROM "subjects" WHERE "subjects"."id" IN (4, 3, 1)
Subject Load (0.5ms) SELECT "subjects".* FROM "subjects" INNER JOIN "qualifications" ON "subjects"."id" = "qualifications"."subject_id" WHERE "qualifications"."teacher_id" = 2
SQL (0.4ms) DELETE FROM "qualifications" WHERE "qualifications"."teacher_id" = 2 AND "qualifications"."subject_id" = 2
TeacherPhone Load (0.4ms) SELECT "teacher_phones".* FROM "teacher_phones" WHERE "teacher_phones"."teacher_id" = 2 LIMIT 1
TeacherEducation Load (0.4ms) SELECT "teacher_educations".* FROM "teacher_educations" WHERE "teacher_educations"."teacher_id" = 2 LIMIT 1
(24.1ms) COMMIT
Redirected to http://0.0.0.0:3000/teachers
Completed 302 Found in 57ms (ActiveRecord: 27.2ms)

Answer 1

Try:
:dependent => :destroy
instead of :dependent => :delete_all

Answer 2

Not sure if this is what you are doing but I'm able to delete dependents using :dependent => :delete_all
Instead of using delete, I used destroy in my controller.
Example:
Instead of:
Qualification.delete(params[:id])
Use this:
Qualification.destroy(params[:id])
Hope this helps. =)

Answer 3

the "the rails 3 way" says that -
(pg 187)
clear
Transactionally removes all records from this association by clearing the foreign key
field (see delete). If the association is configured with the :dependent option set to
:delete_all, then it calls delete_all. Similarly, if the :dependent option is set to
:destroy_all, then the destroy_all method is invoked.
delete(*records) and delete—all
The delete and delete_all methods are used to sever specified associations, or all of
them, respectively. Both methods operate transactionally.
It’s worth noting, for performance reasons, that calling delete_all first loads the
entire collection of associated objects into memory in order to grab their ids. Then it
executes a SQL UPDATE that sets foreign keys for all currently associated objects to nil,
effectively disassociating them from their parent. Since it loads the entire association into memory, it would be ill-advised to use this method with an extremely large collection of associated objects.
Note
The names of the delete and delete_all methods can be misleading. By default, they
don’t delete anything from the database—they only sever associations by clearing the foreign
key field of the associated record. This behavior is related to the :dependent option, which
defaults to :nullify. If the association is configured with the :dependent option set to
:delete or :destroy, then the associated records will actually be deleted from the database.
:dependent => :destroy or :delete
Specifies a rule that the associated owner record should be destroyed or just deleted from
the database, depending on the value of the option. When triggered, :destroy will call
the dependent’s callbacks, whereas :delete will not.
Usage of this option might make sense in a has_one / belongs_to pairing. However,
it is really unlikely that you want this behavior on has_many / belongs_to relationship;
it just doesn’t seem to make sense to code things that way. Additionally, if the owner
record has its :dependent option set on the corresponding has_many association, then
destroying one associated record will have the ripple effect of destroying all of its siblings.