I'm considering this an add-on question of sorts to the thread below:
Using join tables in ruby on rails
So we have 'Student' and 'Course' scaffolds joined by a many-to-many association, but in addition there is also a 'Semester' scaffold and what I wish to do is, for each student that is added to a course, for the application to search for previous iterations of the same course through past semesters, to that it's known how many times a student has taken that class before. I'm kind of mixed up at the moment as to how to implement this, so I was hoping someone could help me pin down the logic and code I should be operating by.
Some underlying assumptions I have so far:
'Course' and 'Semester' should, like 'Student' and 'Course', be joined
by a many-to-many association (many courses are taught per semester,
and a course is taught for more than one semester).
There should be an action (let's say get_student) within the course
controller to locate the student via student_id. This would be the main area I'm scratching my head as to what to do. What would this method look like in code?
Within the student-course join table I should have an attribute
'attempts' which increments each time get_student finds this
student_id combined with the course_id that calls the method.This
would be the mechanism that actually tells how many times the course
had been attempted by the student.
I initially wondered if there should be a 'semester' controller
action to activate get_student across all semesters, but now I'm
thinking that get_student should work fine without that.
Appreciate any help I can get on this. Thanks.
This is not a good answer, just a comment.
I would comment, but hear will be more clear. I ll update for the other points. This is just an ongoing feedback/discussion, not an answer.
class Semester < ApplicationRecord
has_many :courses
end
class Course < ApplicationRecord
has_many :students
end
And
semester.courses[0].students => outputs the students array for that
This could be the method to calculate the number of student that did that course:
def studentForCourse
#input_params.course_id => course id you are selecting
semester = Semester.find(input_params)
semester.courses.each do |course|
if course.id = input_params.course_id
nstudents = course.students.size
end
end
Related
I'm an experienced programmer who is relatively new to ruby/rails and databases. I have created a large website for signing up for courses. I thought I was being clever creating categories of course signups using scope and has_many. Logically, it encapsulates the information well, but my pages are now loading super-slow, and trying to eager load is confusing me.
I have models for Course, Person, CourseRole (student, teacher, etc.), and CourseSignup which includes one of each (Course, Person, CourseRole). It all works smoothly. Recently, I set up scopes in CourseRole to define the categories of signups (I had been hardcoding the role name, and wanted to get away from that). I then set up has_many relationships in Course for each of the categories. So, Course inherently has_many course_signups, and has my categories
has_many :student_signups, -> { CourseSignup.student }, class_name: 'CourseSignup', foreign_key: :course_id
has_many :teacher_signups, -> { CourseSignup.teacher }, class_name: 'CourseSignup', foreign_key: :course_id
etc. (I have 6 categories). I have a page that lists all courses and all of the signups for each course. Like:
Dodge Ball:
info about course
Students:
names of students
Teachers:
names of teachers
etc.
This page loads incredibly slowly. I was trying to add includes statements to the query (based on recommendations from the Bullet gem), but it actually makes it slower. This leads me to think I'm making this more complicated than I should, but I don't know enough to have a clue how to fix it. I imagine I should restructure my models. But I like the abstraction of the scopes/has_many
The page is generated by looping through rendering a partial which shows one course.
#courses = #cuco_session.assigned_courses.includes(:period).order('periods.start_time')
works but is very slow.
#courses = #cuco_session.assigned_courses
.includes(course_signups: [:person, :course_role])
.includes([:courses_rooms, :rooms])
.includes([:helper_signups, :student_signups, :volunteer_signups, :waiting_list_signups, :person_in_room_signups])
.includes(:period).order('periods.start_time')
Also works but is even slower.
We need more information about what is actually happening in the view to really answer why your page is slow.
Also a snippet of your log showing the actual SQL would be necessary.
You mention being new to using databases online with Rails. It's hard to give advice without more of a look into the structure of some of these models, but have you considered replacing CourseSignup with just a join table between Course and Person? It seems like that's what you're getting at here but it's hard to tell.
*has_many :student_signups, -> { CourseSignup.student }, class_name: 'CourseSignup', foreign_key: :course_id
has_many :teacher_signups, -> { CourseSignup.teacher }, class_name: 'CourseSignup', foreign_key: :course_id*
above code can be refactored using Single table inheritance
# app/models/person.rb
class Person < ApplicationRecord
#your code
end
# app/models/teacher.rb
class Teacher < Person
#your code
end
# app/models/student.rb
class Student < Person
#your code
end
For Course Role you can have a separate master table of Role and put references of Person table and Role table in Course Role table
I hope above solution put some light on your question.
This is probably a really simple question, but I've been searching the web for probably around an hour and I can't really find an answer to my problem. It should be clear by what follows that I am very new to Rails, so my terminology and explanation might be a bit confusing.
Let's say that I were making a social media app on Rails, where one of the models is User. I want to make a many-to-many relationship called "friends", which links two users together. Let's say in this situation I also wanted to make a many-to-many between two users called "enemies".
This is all completely hypothetical, but the idea is the same one that I want to use for something I'm working on.
Because a user can have many friends and enemies, but also be many friends and enemies, I would use:
class User < ActiveRecord::Base
has_and_belongs_to_many :users #this should be the friends association
has_and_belongs_to_many :users #this should be the enemies association
end
Now I'm guessing I can't just do that, because I would have to have two tables both named users_users. So, then I switch to:
class User < ActiveRecord::Base
has_and_belongs_to_many(:users, join_table: 'friends',
foreign_key: 'user_id', associate_foreign_key: 'friend_id')
end
With a similar statement for the enemies table. Now, my problem is that I want to have a form that the user can use when they sign up, where they can input their information (this is the User object details), and also list their friends and enemies.
Because the user won't have the database id key for their friends or enemies, they'll have to input the users' names. This is fine, though because the name is also a unique key, guaranteed by the validation.
However, if the user types in the name of a friend, I can't join the two if the friend happens to not exist. So, I use a custom validation class that looks something like this:
class FriendValidator < ActiveModel::Validator
def validate(object)
#lookup user and throw error if not found.
end
end
which will access the variable (object.friends) and (object.enemies)
With something similar for enemies. So therefore, above my has_and_belongs_to_many statements, I have lines that say:
attr_accessor :friends, :enemies #these are attrs because they don't exist within the model's db
validates_with FriendValidator
When I create the form with erb, I have the standard form_for block
<%= form_for(#user) do |f| %>
It seems to me that I can't just stick
<%= f.text_area :friends %>
because friends isn't actually something that will get passed to the User object, but rather a separate table. (Can I, though? Because the attr_accessor is declared in the user's model class?)
So now, we have my main problem. I have two many-to-many tables with a model to its own model class, and I don't know how to ensure that the validation class will take the two attributes, lookup and throw necessary errors, and then add a row to the join tables using the id of the user, rather than the string inputted. What form fields should I use to pass the input to the right place? Where do I change the controller methods so that the input gets sent to the join table rather than the user object?
This definitely seems like a pretty specific situation, so I can't really find an answer in the Rails documentation, which I've been learning from.
My initial impression of this problem has to do with your associations. To me, a user has_many enemies and has_many friends.
friends belong_to user
enemies belong_to user
Not sure if a many to many relationship makes sense in this case. Maybe that's why you are having such a hard time finding an answer online. Just my two cents.
When an entity isn't associated with many other entity at the same time, should I still use many-to-many association?
For example, I understand that Author-Book relationship is a many to many, An author can write many books, a book can be written by many authors. This holds for all time.
Consider this situation. A Batch has many students at one particular time. If a student fails, he should move to the next junior batch. In other word, A student cannot belong to more than one batch at the same time. In this case, is this a many to many association?
Alternative Solution I thought:
I was thinking about putting two columns in students table. initial_batch and current_batch. I can get the student's Batch history, by checking the gaps between current and initial batch, because of the rule A student must drop to the next immediate batch. Also, batch.students exists with a has_many association in rails. So, I think that would not be a big deal.
By Batch, I mean batch of students get admitted in the same year.
I think you'd still want to use an association table (many-to-many association) because you might want to keep a history of which classes the student has taken.
If you want it to be a many-to-one and have a student belong to one class at a time, you'd have to add a class_id field to your student table. This allows you to call student.class to find their current class, but you wouldn't be able to do class.students because that relation doesn't exist. For this reason, I think it should be a many-to-many.
is this a many to many association
Yep.
An ActiveRecord association is simply a way for you to create two connected objects (ActiveRecord is an ORM -- Object Relationship Mapper).
Remember, as Ruby is object orientated, each "Model" (class) is counted as an object, invoked every time you want to populate it with data. Each ActiveRecord association for each object is accessed through a method in the object...
#app/models/student.rb
class Student < ActiveRecord::Base
has_many :subjects #-> AR appends the "subjects" method to your Student object
end
Simply, this means that even if you have the functionality for a number of different objects, you don't need to have the method populated with data.
If you have the following...
#app/models/student.rb
class Student < ActiveRecord::Base
has_and_belongs_to_many :subjects
end
#app/models/subject.rb # "class" is a reserved word
class Subject < ActiveRecord::Base
has_and_belongs_to_many :students
end
This simply provides the functionality / capacity for #students.subjects etc.
What's contained inside this method is up to you to determine:
<% if #students.subjects.any? %>
<% #students.subjects.each do |subject| %>
...
<% end %>
<% end %>
I have groups (Group model) in my app, which represent groups of people.
I want each group to have its own forum.
Should I just have the forum id in the groups table? It doesn't feel right. If I did it myself, the forum would have a polymorphic association to a "forumable" element (groups in this case, but I have other models that would need a forum).
Any opinions on what I should do? Modify the gem to fit my needs, or just have the forum_id in my models that need a forum? Or another solution maybe?
I'm the guy who started Forem (its the volunteers who did most of the hard work, though!), I think I can answer this question.
If you want only certain groups to have access to one and only one forum then you can put the forum_id field on the groups table and do it that way. What you can do then is override the can_read_forem_forum? method in your User model to perform a permission check for that user:
def can_read_forem_forum?(forum)
groups.where(:forum_id => forum.id).any?
end
This is used in Forem's ability model to determine whether or not a person can access a forum. What this method is going to do is that it will only return groups for that user that have link that specific forum. If there are any, then it's known that the user can access that forum.
Now if you're going the other route where a group may have access to many forums, well then you'd define a joins table between groups and forem_forums (called forum_groups) and define it as an association in your Group model like this:
has_many :forum_groups
has_many :forums, :through => :forum_groups, :class_name => "Forem::Forum"
You would need to also define a new model inside your application for this forum_groups association, it would be called ForumGroup and go a little like this:
class ForumGroup < ActiveRecord::Base
belongs_to :forum, :class_name => "Forem::Forum"
belongs_to :group
end
We're doing it this way so you have an easy way to manage the associations between forums and groups. If you did has_and_belongs_to_many, it generally only provides a gigantic pain in the ass when you want to delete one specific record from that join table.
Now, with that all nicely set up, the method you want to define in your User model is this one:
def can_read_forem_forum?(forum)
groups.joins(:forums).where("forem_forums.id = ?", forum.id).any?
end
Same thing, except this time we find all the groups that are linked to a specific forum through that association we set up earlier. This will do an INNER JOIN on the forum_groups table, and then another on the forem_forums table, getting the data required.
I hope this helps you, and thanks for using Forem!
I have three tables/models. User, Alliance and Alliance_Membership. The latter is a join table describing the :Alliance has_many :Users through :Alliance_Membership relationship. (:user has one :alliance)
Everything works ok, but Alliance_Membership now has an extra field called 'rank'. I was thinking of the best way to access this little piece of information (the rank).
It seems that when i do "alliance.users", where alliance is the user's current alliance object, i get all the users information, but i do not get the rank as well. I only get the attributes of the user model. Now, i can create a helper or function like getUserRole to do this for me based on the user, but i feel that there is a better way that better works with the Active Record associations. Is there really a better way ?
Thanx for reading :)
Your associations are all wrong - they shouldn't have capital letters. These are the rules, as seen in my other answer where i told you how to set this up yesterday :)
Class names: Always camelcase like AllianceMembership (NOT Alliance_Membership!)
table names, variable names, methods and associations: always underscored and lower case:
has_many :users, :through => :alliance_memberships
To find the rank for a given user of a given alliance (held in #alliance and #user), do
#membership = #alliance.alliance_memberships.find_by_user_id(#user.id)
You could indeed wrap this in a method of alliance:
def rank_for_user(user)
self.alliance_memberships.find_by_user_id(user.id).rank
end