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Bruno Amaral

2010-Nov-28 21:07 UTC

querying has_many relationships

I am trying to build a small database of Universities, Departments and
Courses. But for some reason I can''t seem to get a list of courses
attached to a Department.

So far I have the following Database:
=> Course(id: integer, course_name: string, length: integer,
description: text, programme: string, department_id: integer,
university_id: integer, created_at: datetime, updated_at: datetime)

=> Department(id: integer, department_name: string,
department_description: text, head_firstname: string, head_lastname:
string, head_email: string, university_id: integer, courses_id:
integer, created_at: datetime, updated_at: datetime)

=> University(id: integer, name: string, website: string, country:
string, city: string, departments_id: integer, courses_id: integer,
created_at: datetime, updated_at: datetime)

Models:
class Course < ActiveRecord::Base
	belongs_to :department
	belongs_to :university
end
class Department < ActiveRecord::Base
  belongs_to :university
  has_many :courses
end
class University < ActiveRecord::Base
  has_many :departments
  has_many :courses
end

For the Department I am trying to run the following controller
  def index
    @departments = Department.includes(:courses, :university)

and the following view:
<% @departments.each do |department| %>
  <tr>
    <td><%= department.department_name %></td>
    <td><%= department.department_description %></td>
    <td><%= department.head_firstname %></td>
    <td><%= department.head_lastname %></td>
    <td><%= department.head_email %></td>
    <td><%= 	department.university.name %></td>
    <td><%= department.courses %>

that last line outputs #<Course:0x1042b8270> and it fails when i try
to add something like department.courses.course_name

I am at a loss here and haven''t been able to find any clues on the
documentation as to what I might be doing wrong. If anyone could at
least point me in the right direction, that would be great.

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Frederick Cheung

2010-Nov-28 21:43 UTC

head link

Re: querying has_many relationships

On Nov 28, 9:07 pm, Bruno Amaral
<m...-S2ydJG5fPKHivB4pSPl/wg@public.gmane.org> wrote:
>     <td><%= department.courses %>
>
> that last line outputs #<Course:0x1042b8270> and it fails when i try
> to add something like department.courses.course_name
>
> I am at a loss here and haven''t been able to find any clues on the
> documentation as to what I might be doing wrong. If anyone could at
> least point me in the right direction, that would be great.
Because department.courses is a collection of courses and doesn''t have
a course_name attribute (although the objects in that collection will
do). You either need to pick which course you want to manipulate all
iterate over them (similarly, you can''t do
@departments.department_name - you had to iterate over the departments
in that collection)

Fred

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skt

2010-Nov-28 21:46 UTC

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Re: querying has_many relationships

I suspect department.courses returns and array and when you print it
out you get the address of the first entry in the array.

That would also explain department.courses.course_name would fail
because you have to index into the department.courses array like
department.courses[x].course_name to be able to set it


On Nov 28, 1:07 pm, Bruno Amaral
<m...-S2ydJG5fPKHivB4pSPl/wg@public.gmane.org>
wrote:> I am trying to build a small database of Universities, Departments and
> Courses. But for some reason I can''t seem to get a list of courses
> attached to a Department.
>
> So far I have the following Database:
> => Course(id: integer, course_name: string, length: integer,
> description: text, programme: string, department_id: integer,
> university_id: integer, created_at: datetime, updated_at: datetime)
>
> => Department(id: integer, department_name: string,
> department_description: text, head_firstname: string, head_lastname:
> string, head_email: string, university_id: integer, courses_id:
> integer, created_at: datetime, updated_at: datetime)
>
> => University(id: integer, name: string, website: string, country:
> string, city: string, departments_id: integer, courses_id: integer,
> created_at: datetime, updated_at: datetime)
>
> Models:
> class Course < ActiveRecord::Base
>         belongs_to :department
>         belongs_to :university
> end
> class Department < ActiveRecord::Base
>   belongs_to :university
>   has_many :courses
> end
> class University < ActiveRecord::Base
>   has_many :departments
>   has_many :courses
> end
>
> For the Department I am trying to run the following controller
>   def index
>     @departments = Department.includes(:courses, :university)
>
> and the following view:
> <% @departments.each do |department| %>
>   <tr>
>     <td><%= department.department_name %></td>
>     <td><%= department.department_description %></td>
>     <td><%= department.head_firstname %></td>
>     <td><%= department.head_lastname %></td>
>     <td><%= department.head_email %></td>
>     <td><%=    department.university.name %></td>
>     <td><%= department.courses %>
>
> that last line outputs #<Course:0x1042b8270> and it fails when i try
> to add something like department.courses.course_name
>
> I am at a loss here and haven''t been able to find any clues on the
> documentation as to what I might be doing wrong. If anyone could at
> least point me in the right direction, that would be great.
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Colin Law

2010-Nov-28 21:48 UTC

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Re: querying has_many relationships

On 28 November 2010 21:07, Bruno Amaral
<mail-S2ydJG5fPKHivB4pSPl/wg@public.gmane.org>
wrote:> I am trying to build a small database of Universities, Departments and
> Courses. But for some reason I can''t seem to get a list of courses
> attached to a Department.
>
> So far I have the following Database:
> => Course(id: integer, course_name: string, length: integer,
> description: text, programme: string, department_id: integer,
> university_id: integer, created_at: datetime, updated_at: datetime)
>
> => Department(id: integer, department_name: string,
> department_description: text, head_firstname: string, head_lastname:
> string, head_email: string, university_id: integer, courses_id:
> integer, created_at: datetime, updated_at: datetime)
>
> => University(id: integer, name: string, website: string, country:
> string, city: string, departments_id: integer, courses_id: integer,
> created_at: datetime, updated_at: datetime)
>
> Models:
> class Course < ActiveRecord::Base
>        belongs_to :department
>        belongs_to :university
You don''t need course belongs_to university, because course belongs_to
department and department belongs_to university.  So the university
for a course is course.department.university.  That is on the
assumption that the university the course belongs to is the same one
that it''s department belongs to.
> end
> class Department < ActiveRecord::Base
>  belongs_to :university
>  has_many :courses
> end
> class University < ActiveRecord::Base
>  has_many :departments
>  has_many :courses
> end
>
> For the Department I am trying to run the following controller
>  def index
>    @departments = Department.includes(:courses, :university)
>
> and the following view:
> <% @departments.each do |department| %>
>  <tr>
>    <td><%= department.department_name %></td>
>    <td><%= department.department_description %></td>
>    <td><%= department.head_firstname %></td>
>    <td><%= department.head_lastname %></td>
>    <td><%= department.head_email %></td>
>    <td><%=     department.university.name %></td>
>    <td><%= department.courses %>
>
> that last line outputs #<Course:0x1042b8270> and it fails when i try
> to add something like department.courses.course_name
I see Fred has pointed out the problem here already.

Colin
>
> I am at a loss here and haven''t been able to find any clues on the
> documentation as to what I might be doing wrong. If anyone could at
> least point me in the right direction, that would be great.
>
> --
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>
>
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Bruno Amaral

2010-Nov-28 22:08 UTC

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Re: querying has_many relationships

Skt, Fred, Collin, thank you!

The following code was enough to get the view working:

 <% department.courses.each do |course| %>
 <%= course.course_name %>
 <% end %>

I am very new to both ruby and rails so I tend to miss a few key
concepts, as was the case.

Thank you again :)


On Nov 28, 9:48 pm, Colin Law
<clan...-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>
wrote:> On 28 November 2010 21:07, Bruno Amaral
<m...-S2ydJG5fPKHivB4pSPl/wg@public.gmane.org> wrote:
>
>
>
>
>
> > I am trying to build a small database of Universities, Departments and
> > Courses. But for some reason I can''t seem to get a list of
courses
> > attached to a Department.
>
> > So far I have the following Database:
> > => Course(id: integer, course_name: string, length: integer,
> > description: text, programme: string, department_id: integer,
> > university_id: integer, created_at: datetime, updated_at: datetime)
>
> > => Department(id: integer, department_name: string,
> > department_description: text, head_firstname: string, head_lastname:
> > string, head_email: string, university_id: integer, courses_id:
> > integer, created_at: datetime, updated_at: datetime)
>
> > => University(id: integer, name: string, website: string, country:
> > string, city: string, departments_id: integer, courses_id: integer,
> > created_at: datetime, updated_at: datetime)
>
> > Models:
> > class Course < ActiveRecord::Base
> >        belongs_to :department
> >        belongs_to :university
>
> You don''t need course belongs_to university, because course
belongs_to
> department and department belongs_to university.  So the university
> for a course is course.department.university.  That is on the
> assumption that the university the course belongs to is the same one
> that it''s department belongs to.
>
>
>
>
>
> > end
> > class Department < ActiveRecord::Base
> >  belongs_to :university
> >  has_many :courses
> > end
> > class University < ActiveRecord::Base
> >  has_many :departments
> >  has_many :courses
> > end
>
> > For the Department I am trying to run the following controller
> >  def index
> >    @departments = Department.includes(:courses, :university)
>
> > and the following view:
> > <% @departments.each do |department| %>
> >  <tr>
> >    <td><%= department.department_name %></td>
> >    <td><%= department.department_description %></td>
> >    <td><%= department.head_firstname %></td>
> >    <td><%= department.head_lastname %></td>
> >    <td><%= department.head_email %></td>
> >    <td><%=     department.university.name %></td>
> >    <td><%= department.courses %>
>
> > that last line outputs #<Course:0x1042b8270> and it fails when i
try
> > to add something like department.courses.course_name
>
> I see Fred has pointed out the problem here already.
>
> Colin
>
>
>
>
>
> > I am at a loss here and haven''t been able to find any clues
on the
> > documentation as to what I might be doing wrong. If anyone could at
> > least point me in the right direction, that would be great.
>
> > --
> > You received this message because you are subscribed to the Google
Groups "Ruby on Rails: Talk" group.
> > To post to this group, send email to
rubyonrails-talk-/JYPxA39Uh5TLH3MbocFF+G/Ez6ZCGd0@public.gmane.org
> > To unsubscribe from this group, send email to
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athttp://groups.google.com/group/rubyonrails-talk?hl=en.
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Julian Leviston

2010-Nov-29 01:00 UTC

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Re: Re: querying has_many relationships

It''d be interesting if AssociationProxy added in pluralised attribute
names as easy go-to''s for mappings, hey?

for example:

department.courses.course_names

(yields the same result as:...)

department.courses.map{|course| course.course_name}


----------------------------------------------
Buy my new album! 16 Sep 2010:
http://itunes.apple.com/au/album/erste-zeit/id393326346







On 29/11/2010, at 8:46 AM, skt wrote:
> I suspect department.courses returns and array and when you print it
> out you get the address of the first entry in the array.
> 
> That would also explain department.courses.course_name would fail
> because you have to index into the department.courses array like
> department.courses[x].course_name to be able to set it
> 
> 
> On Nov 28, 1:07 pm, Bruno Amaral
<m...-S2ydJG5fPKHivB4pSPl/wg@public.gmane.org> wrote:
>> I am trying to build a small database of Universities, Departments and
>> Courses. But for some reason I can''t seem to get a list of
courses
>> attached to a Department.
>> 
>> So far I have the following Database:
>> => Course(id: integer, course_name: string, length: integer,
>> description: text, programme: string, department_id: integer,
>> university_id: integer, created_at: datetime, updated_at: datetime)
>> 
>> => Department(id: integer, department_name: string,
>> department_description: text, head_firstname: string, head_lastname:
>> string, head_email: string, university_id: integer, courses_id:
>> integer, created_at: datetime, updated_at: datetime)
>> 
>> => University(id: integer, name: string, website: string, country:
>> string, city: string, departments_id: integer, courses_id: integer,
>> created_at: datetime, updated_at: datetime)
>> 
>> Models:
>> class Course < ActiveRecord::Base
>>         belongs_to :department
>>         belongs_to :university
>> end
>> class Department < ActiveRecord::Base
>>   belongs_to :university
>>   has_many :courses
>> end
>> class University < ActiveRecord::Base
>>   has_many :departments
>>   has_many :courses
>> end
>> 
>> For the Department I am trying to run the following controller
>>   def index
>>     @departments = Department.includes(:courses, :university)
>> 
>> and the following view:
>> <% @departments.each do |department| %>
>>   <tr>
>>     <td><%= department.department_name %></td>
>>     <td><%= department.department_description %></td>
>>     <td><%= department.head_firstname %></td>
>>     <td><%= department.head_lastname %></td>
>>     <td><%= department.head_email %></td>
>>     <td><%=    department.university.name %></td>
>>     <td><%= department.courses %>
>> 
>> that last line outputs #<Course:0x1042b8270> and it fails when i
try
>> to add something like department.courses.course_name
>> 
>> I am at a loss here and haven''t been able to find any clues on
the
>> documentation as to what I might be doing wrong. If anyone could at
>> least point me in the right direction, that would be great.
> 
> -- 
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Marnen Laibow-Koser

2010-Nov-29 01:13 UTC

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Re: Re: querying has_many relationships

Julian Leviston wrote in post #964556:> It''d be interesting if AssociationProxy added in pluralised
attribute
> names as easy go-to''s for mappings, hey?
No.  What would be the point?
>
> for example:
>
> department.courses.course_names
>
> (yields the same result as:...)
>
> department.courses.map{|course| course.course_name}
Why? So you don''t have to think about what you''re doing?  Or
is there a
good reason that escapes me?
>
>
> ----------------------------------------------
> Buy my new album! 16 Sep 2010:
> http://itunes.apple.com/au/album/erste-zeit/id393326346
Cool!  Nice to see other musicians here.

Best,
-- 
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http://www.marnen.org
marnen-sbuyVjPbboAdnm+yROfE0A@public.gmane.org

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Rails - Nov 2010 - querying has_many relationships

querying has_many relationships

Re: querying has_many relationships

Re: querying has_many relationships

Re: querying has_many relationships

Re: querying has_many relationships

Re: Re: querying has_many relationships

Re: Re: querying has_many relationships