I have a problem with habtm on a single model in rails.
Example:
Let us say i have a User model with two roles "Student" and "teacher". User model is common for two roles. Now
Each student can be associated to many teachers
Each teacher can be associated to many students
In rails notation, their should be habtm between teacher and student
How this can be achieved with single table.
Thanks,
Aashish
It can't be done with a single table. In a many-to-many relationship, no matter what, you always need a table where you store the associations.
In your case, given the association seems to be parent/child, then you just need two tables instead of one.
How to implement it, it depends on your database structure and data organization. You should create an users_users table (as part of the habtm) and configure the references accordingly. If the user table, as it seems to be, is also used for STI, then the configuration may change a little bit.
Related
I’m working with Ruby on Rails, but this question applies to application/database model design in general. I want to model many types of people such as User (someone who can log in), Employee, and Customer. Each of these has common attributes like name and email that should be part of a Person superclass. Each type also has other more specific attributes. A Person can be one or more of these roles (someone can be both an employee and a customer).
In code, the appropriate structure seems like a superclass/subclass relationship, but from a database normalization perspective, there should be a Customer table that references the Person table for its common attributes.
I need to decide how to combine these two approaches.
For example, I want to be able to create a Customer by Customer.create(name: “Johnny Appleseed”, favorite_product: widget) and query Employee.where(email: “someone#company.com”), and avoid duplicating Person fields on every role model. That way I can change the way I model a Person without needing to update all the other tables with the same changes.
It would be better if you use Single Table Inheritance.
You can start from this tutorial https://samurails.com/tutorial/single-table-inheritance-with-rails-4-part-1/
I am working on a web app written in rails. It is currently running on heroku with a postgres database.
I am supposed to add a feature where users may enter up to three codes for each one of the user's students. The codes themselves are irrelevant, they are simply strings that will be entered into the database.
This brings me to my dilemma. I am unsure of how to best store the codes in terms of their relationship to the student table. My original thought was to use the rails method serialize to store up to three codes in an array, but I have read that more often than not, storing data in an array in a database is not what you want to do.
Should I create a new table "codes" and set up a has_many relationship with the "students" table? Or is there a more preferable away to set up this relationship?
Given your situation, this sounds like the most reasonable approach to have a Code model and then setup has_many association with Student model.
student has_many codes and
code belongs_to student.
I have HABTM relationships between Topic and Chapter Now I want to display chapters in a specific order under topics.
For Eg: A particular chapter might be 2nd chapter under one topic, and might be 8th chapter under another topic.
How Should I do it? Thanks in advance.
In this case you have a non-trivial many-to-many relationship, the join table needs to contain more than just foreign keys to each of main tables.
As a result, you'll want to move from HABTM to has_many :through on both Topic and Chapter, and change the join table to be its own model that not only relates the two main tables to each other, but also stores the order in a column (which, by the way, you should not name "order", since it will confuse SQL and Rails :-).
Converting from a standard Rails HABTM model is pretty easy, but one bit of advice: think of what it is that makes what used to be just a join table a real model ... and use that as a name for the new model (and associated new table). In a simple HABTM, you would create a table chapters_topics, when it becomes a model it might be "ChapterTopic" (resulting in a table chapter_topics).
I have an interesting problem that I never had to deal with before. I'm looking for the best way to approach it.
I'm creating an admin site linking students to virtual machines. A Student can sign up for multiple courses and has one virtual_machine_id which depends on the combination of courses they take. An Admin can create new courses and (separately) map combinations of courses to specific virtual_machine_ids.
1) What's the best way to associate a combination of variable elements with a specific id? If the elements were fixed, I would create a table with columns for each element and one column for the virtual_machine_id. But since these elements can change (as admins add or remove courses), how do I map them in a way that I can easily query for a combination and it's associated id?
2) Right now, I have Students mapped to Courses using a has_many :through association and a third table with student_id and course_id. Is this the right way if I need to collect combinations of courses and assign the entire combination a single virtual_machine_id (i can't assign the id to the student because they could potentially have more than one virtual_machine depending on how many courses they take)
I was looking at the EAV (entity-attribute-value) model as a solution but the general consensus seems to be that it's a bad a idea because you lose some ActiveRecord features.
If you wish to use EAV with ActiveRecord ORM you can look at hydra_attribute gem which allows to create new attributes in runtime and has possibility to find/sort/group by them.
Currently the 0.3.2 version doesn't support attribute sets but in the next release I'll add this feature and you will be able to assign the unique attribute collection to each Student record.
How do the relationships magically function when only the models are altered?
If I want a "has__and___belongs___to__many" relationship, what should I name the table (so Rails can use it) that contains the two foreign keys?
Short answer: You can't just tell the models that they're related; there have to be columns in the database for it too.
When you set up related models, Rails assumes you've followed a convention which allows it to find the things you wrote. Here's what happens:
You set up the tables.
Following conventions in Rails, you name the table in a particular, predictable way (a plural noun, e.g. people). In this table, when you have a relationship to another table, you have to create that column and name it in another predictable way (e.g. bank_account_id, if you're relating to the bank_accounts table).
You write a model class inheriting from ActiveRecord::Base
class Person < ActiveRecord::Base
When you instantiate one of these models, the ActiveRecord::Base constructor looks at the name of the class, converts it to lowercase and pluralizes it. Here, by reading Person, it yields people, the name of the table we created earlier. Now ActiveRecord knows where to get all the information about a person, and it can read the SQL output to figure out what the columns are.
You add relationships to the model: has_many, belongs_to or has_one.
When you type something like, has_many :bank_accounts, it assumes a few things:
The name of the model that you relate to is BankAccount (from camel-casing :bank_accounts).
The name of the column in the people table which refers to a bank account is bank_account_id (from singularizing :bank_accounts).
Since the relationship is has_many, ActiveRecord knows to give you methods like john.bank_accounts, using plural names for things.
Putting all of that together, ActiveRecord knows how to make SQL queries that will give you a person's bank accounts. It knows where to find everything, because you followed a naming convention that it understands when you created the table and its colums.
One of the neat things about Ruby is that you can run methods on a whole class, and those methods can add other methods to a class. That's exactly what has_many and friends are doing.
This works because you are following "Convention over Configuration".
If you state that a customer model has many orders then rails expects there to be a customer_id field on the orders table.
If you have followed these conventions then rails will use them and will be able to build the necessary SQL to find all the orders for a given customer.
If you look at the development.log file when you are developing your application you will be able to see the necessary SQL being built to select all orders for a given customer.
Rails does not create tables without you asking it to. The creation of tables is achieved by generating a migration which will create/alter tables for you. The fact that you create a customer model and then state within it that it has_many :orders will not create you an orders table. You will need to do that for yourself within a migration to create an orders table. Within that migration you will need to either add a customer_id column or use the belongs_to: customer statement to get the customer_id field added to the orders table.
The rails guide for this is pretty useful