How to update attributes in many to many relation in rails? - ruby-on-rails

Question

I am new in rails. So i am not comprehending much of it. I have a database with three table. students table, courses table and registrations table.
My Railde models are as follows :
class Student < ActiveRecord::Base
attr_accessible :name
has_many :registrations
has_many :courses, :through => :registrations
validates :name, :presence => true
end
class Course < ActiveRecord::Base
attr_accessible :name
has_many :registrations, :dependent => :destroy
has_many :students, :through => :registrations
validates :name , :presence => true
validates :name, :uniqueness => true
end
class Registration < ActiveRecord::Base
attr_accessible :course_id, :student_id
belongs_to :course
belongs_to :student
validates :course_id, :student_id, :presence => true
validates :course_id, :uniqueness => {:scope => :student_id}
validates :student_id, :uniqueness => {:scope => :course_id}
end
......................
Controller action for updating student :
def update
#student = Student.find(params[:id])
if #student.update_attributes(params[:student])
redirect_to students_path, :notice => "Registration completed"
else
render 'edit'
end
end
................
View :
<%=form_for #student do |f| %>
<p>
<%= f.label :name, "Name" %>
<%= f.text_field :name %>
</p>
<p>
<%= render('course_list') %>
</p>
<p>
<%= f.submit %>
</p>
<% end %>
...............
_course_list partial :
Select Courses :<br/>
<p>
<% Course.all.each do |course| %>
<%=check_box_tag "student[course_ids][]", course.id, `enter code here`#student.course_ids.include?(course.id) %>
<%= course.name %> <br/>
<% end %>
</p>
.............................
when i submit the update button, i got an error
Can't mass-assign protected attributes: course_ids
.......
parameters :
{"utf8"=>"✓",
"_method"=>"put",
"authenticity_token"=>"j/lDE5bv1gWkfadQ6Cag6hGjg5nD2Ikad9vHOJTE7Pc=",
"student"=>{"name"=>"Galib",
"course_ids"=>["2",
"3"]},
"commit"=>"Update Student",
"id"=>"6"}
.........................
What i want to do is, if update button is clicked, both students table and registrations table need to be updated. Please help.

Answer 1

In models define the associations
class Student < ActiveRecord::Base
attr_accessible :name
has_many :registrations
has_many :courses, :through => :registrations
validates :name, :presence => true
accepts_nested_attributes_for :courses
attr_accessible :course_ids
end
class Course < ActiveRecord::Base
attr_accessible :name
has_many :registrations, :dependent => :destroy
has_many :students, :through => :registrations
validates :name , :presence => true
validates :name, :uniqueness => true
accepts_nested_attributes_for :students
end
class Registration < ActiveRecord::Base
attr_accessible :course_id, :student_id
belongs_to :course
belongs_to :student
validates :course_id, :student_id, :presence => true
validates :course_id, :uniqueness => {:scope => :student_id}
validates :student_id, :uniqueness => {:scope => :course_id}
accepts_nested_attributes_for :course
end
In Controller
def update
#student = Student.find(params[:id])
if #student.update_attributes(params[:student])
#student.courses.build
redirect_to students_path, :notice => "Registration completed"
else
render 'edit'
end
end
It may help you

Answer 2

The error is telling you that you have a permission error - course_ids can't be posted to from a form. More specifically, you have attr_accessible :name in the Student model which means that is the only attribute that can be saved when you save a Student record using a form (attr_accessible dictates what can be mass assigned).
Try changing your Student model to:
attr_accessible :name, :registrations_attributes
accepts_nested_attributes_for :registrations
You are trying to created a nested model form, so accepts_nested_attributes_for is used for this.
If you want to have the Registrations table updated, as well, you're going to have to tell the partial so that it knows that the partial is updating a different model than Student.
<%= f.fields_for :registrations do |registration| %>
<%= render('course_list') %>
<% end %>

Answer 3

In case of MongoDB, For implementing many to many in rails model you have to use has_and_belongs_to_many instead of simple has_many