Rails - Apr 2013 - How to fit an ER model into Rails? (help an old procedural programmer)

Hi guys!

  I am an old procedural programmer learning Rails and object
orientation.  :)

  I wish to create a little system to help my wife who is a teacher.

  The attached PDF file has the preliminary Entity-Relationship model.

  I am having a rough time trying to put this model into Rails...  :(

  Looking at the model I can think about the SQL code necessary.  But I
think Ruby and Rails is here to help and speed the development process.
;)

  For some examples:

    Who teaches at school "x"?

SELECT people.name FROM people WHERE people.id IN
  (SELECT team_has_teachers.person_id FROM team_has_teachers WHERE
team_has_teachers.team_id IN
    (SELECT teams.id AS team_id FROM teams WHERE teams.school_id       (SELECT
schools.id AS school_id FROM schools WHERE schools.name ''x'')))


    Who learns from teacher "x"?

SELECT people.name FROM people WHERE people.id IN
  (SELECT team_has_students.person_id FROM team_has_students WHERE
team_has_students.team_id IN
    (SELECT team_has_teachers.team_id FROM team_has_teachers WHERE
team_has_teachers.person_id       (SELECT people.id FROM people WHERE
people.name = ''x'')))


    Who is in team "x"?

SELECT people.name FROM people WHERE people.id IN
  (SELECT team_has_students.person_id FROM team_has_students WHERE
team_has_students.team_id IN
    (SELECT teams.id FROM teams WHERE teams.name = ''x''))


  Any help is welcome!


Best regards,
  Mosoleu

Attachments:
http://www.ruby-forum.com/attachment/8356/ER_model.pdf


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On 19 April 2013 14:45, Leandro Peracchi
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Hi guys!
>
>   I am an old procedural programmer learning Rails and object
> orientation.  :)
>
>   I wish to create a little system to help my wife who is a teacher.
>
>   The attached PDF file has the preliminary Entity-Relationship model.
>
>   I am having a rough time trying to put this model into Rails...  :(
>
>   Looking at the model I can think about the SQL code necessary.  But I
> think Ruby and Rails is here to help and speed the development process.
It is rare to have to use SQL directly in Rails.  If you have to then
you are probably doing something wrong.
> ;)
>
>   For some examples:
>
>     Who teaches at school "x"?
@school.teachers
>
> SELECT people.name FROM people WHERE people.id IN
>   (SELECT team_has_teachers.person_id FROM team_has_teachers WHERE
> team_has_teachers.team_id IN
>     (SELECT teams.id AS team_id FROM teams WHERE teams.school_id >      
(SELECT schools.id AS school_id FROM schools WHERE schools.name >
''x'')))
>
>
>     Who learns from teacher "x"?
@teacher.pupils
though in pra
>
> SELECT people.name FROM people WHERE people.id IN
>   (SELECT team_has_students.person_id FROM team_has_students WHERE
> team_has_students.team_id IN
>     (SELECT team_has_teachers.team_id FROM team_has_teachers WHERE
> team_has_teachers.person_id >       (SELECT people.id FROM people WHERE
people.name = ''x'')))
>
>
>     Who is in team "x"?
@team.members
>
> SELECT people.name FROM people WHERE people.id IN
>   (SELECT team_has_students.person_id FROM team_has_students WHERE
> team_has_students.team_id IN
>     (SELECT teams.id FROM teams WHERE teams.name = ''x''))
>
>
>   Any help is welcome!
The details are not correct as I have not looked at the ER diagram,
and it is arguable whether you should start with such a diagram in the
first place, but in order to understand the basics of what rails can
do for you I suggest you work right through a good tutorial such as
railstutorial.org, which is free to use online.  Also look at the
Rails Guides, particularly ActiveRecord Associations.

Colin

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Hi Colin!

  Thanks for your tips.  :)

  With help of Rails Guides on ActiveRecord Associations I reach this 
point:

= Migrations 
class CreateSchools < ActiveRecord::Migration
  def change
    create_table :schools do |t|
      t.string :name

      t.timestamps
    end
  end
end

class CreateTeams < ActiveRecord::Migration
  def change
    create_table :teams do |t|
      t.integer :school_id
      t.string :name

      t.timestamps
    end
  end
end

class CreatePeople < ActiveRecord::Migration
  def change
    create_table :people do |t|
      t.string :name

      t.timestamps
    end
  end
end

class CreateTeachers < ActiveRecord::Migration
  def change
    create_table :teachers, :id => false do |t|
      t.integer :team_id
      t.integer :person_id

    end
    add_index :teachers, [:team_id, :person_id]
  end
end

class CreateStudents < ActiveRecord::Migration
  def change
    create_table :students, :id => false do |t|
      t.integer :team_id
      t.integer :person_id

    end
    add_index :students, [:team_id, :person_id]
  end
end


= Models 
class School < ActiveRecord::Base
  has_many :teams
end

class Team < ActiveRecord::Base
  belongs_to :school

  has_many :teachers
  has_many :people, :through => :teachers

  has_many :students
  has_many :people, :through => :students
end

class Person < ActiveRecord::Base
end

class Teacher < ActiveRecord::Base
  belongs_to :team
  belongs_to :person
end

class Student < ActiveRecord::Base
  belongs_to :team
  belongs_to :person
end

==
  But after put some test data into the tables I cannot get, for 
example, the names os the students of some team or what are the teachers 
that taught some team.

  Look what I tried:

samsara:first_app leandro$ rails console
Loading development environment (Rails 4.0.0.beta1)
2.0.0-p0 :001 > team = Team.find_by_name("Turma 2A")
  Team Load (0.9ms)  SELECT "teams".* FROM "teams" WHERE
"teams"."name"
= ''Turma 2A'' LIMIT 1
 => #<Team id: 4, school_id: 4, name: "Turma 2A", created_at: 
"2013-04-19 12:42:47", updated_at: "2013-04-19 12:42:47">
2.0.0-p0 :002 > team.teachers
  Teacher Load (1.6ms)  SELECT "teachers".* FROM "teachers"
WHERE
"teachers"."team_id" = ?  [["team_id", 4]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Teacher team_id:
4,
person_id: 2>, #<Teacher team_id: 4, person_id: 15>]>
2.0.0-p0 :003 > team.students
  Student Load (0.2ms)  SELECT "students".* FROM "students"
WHERE
"students"."team_id" = ?  [["team_id", 4]]
 => #<ActiveRecord::Associations::CollectionProxy [#<Student team_id:
4,
person_id: 8>, #<Student team_id: 4, person_id: 9>, #<Student
team_id:
4, person_id: 11>, #<Student team_id: 4, person_id: 12>]>
2.0.0-p0 :004 >




  I would like to get the People objects instead of the intermediate 
associative tables "teachers" and "students".  ;)


Best regards,
  Mosoleu

Colin Law wrote in post #1106292:
> On 19 April 2013 14:45, Leandro Peracchi
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>
> The details are not correct as I have not looked at the ER diagram,
> and it is arguable whether you should start with such a diagram in the
> first place, but in order to understand the basics of what rails can
> do for you I suggest you work right through a good tutorial such as
> railstutorial.org, which is free to use online.  Also look at the
> Rails Guides, particularly ActiveRecord Associations.
>
> Colin
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On 19 April 2013 16:09, Leandro Peracchi
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Hi Colin!
>
>   Thanks for your tips.  :)
> ...
> = Models >
> class School < ActiveRecord::Base
>   has_many :teams
> end
>
> class Team < ActiveRecord::Base
>   belongs_to :school
>
>   has_many :teachers
>   has_many :people, :through => :teachers
>
>   has_many :students
>   has_many :people, :through => :students
> end
>
> class Person < ActiveRecord::Base
> end
>
> class Teacher < ActiveRecord::Base
>   belongs_to :team
>   belongs_to :person
> end
>
> class Student < ActiveRecord::Base
>   belongs_to :team
>   belongs_to :person
> end
I would not do it like that.  I would just have a people table that
contains all the students and teachers, without the separate classes.
Look up self referential associations to see how you can then have
@person.students (which will be people) and @person.teacher.  If you
want the classes then you could use STI.

Colin

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>
> class Team < ActiveRecord::Base 
>   belongs_to :school 
>   has_many :teachers 
>   has_many :people, :through => :teachers 
>   has_many :students 
>   has_many :people, :through => :students 
> end 

I''m pretty sure your has_many :people won''t work, because you
have two of
them with different :through clauses.  You would have to do something like:

has_many :teacher_people, :class_name => ''People'', :through
=> :teachers
has_many :student_people, :class_name => ''People'', :through
=> :students

But as you can see, the naming gets awkward pretty quickly, as well as your 
database.  It would be better to do as Colin suggested, and simply have a 
People table that contains all the teachers and students.  If you really 
want separate classes for the teachers and students, you could use single 
table inheritance, but it may not be necessary.  Then you could simple have 
a team_members table that linked your teams to people. 

class Team < ActiveRecord::Base 
  belongs_to :school 
  has_many :team_members
  has_many :members, :through => :team_members
end 

class Person < ActiveRecord::Base
  scope :teachers, where(:person_type => ''Teacher'')
  scope :students, where(:person_type => ''Student'')
end

Then, you could get members, teachers, and students for teams like this:

team.members
team.members.teachers
team.members.students

Jim
>
>
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