i am fairly new to rails and want to take advantage of the associations ActiveRecord provides in trying to set up a new project and schema. i have two models, users and questions. when a user is being set up by an administrative user, i have a form with checkboxes for a list of categories. i want these categories to map to specific questions to ask the setup user at a later time. my thinking is i have a mapping table, user_to_categories, that will allow me to get from the user to the necessary questions. but the questions table will have a primary key of question_id, so the has_many :through does not seem to work here. since many questions can be in the same category, category_id will just be another column in the questions table. any suggestions on how to set this up to take advantage of rails and active record associations?
Many to Many between user and categories and question belongs to category.
Now when you add user select categories by passing category_ids through UI,
Related
I have a relationship between employee and sale NxM. But in the employye_sale table I have the commission attribute.
The problem is that! I can not take this commission field and show it in the view.
For example: When registering employee, I can already add a sale in the same view using rails_admin. But the commission field is not possible to access.
I'm using the gem rails_admin, using inverse_of and accepts_nested_attributes_for. Where it already does the work of joining the view of employee into sale. I still can not get a good way to put the relationship attributes
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.
Let me try and condense my question:
I want to display data from multiple tables in a particular view. I want to list every person I have in a "People" table, and append there job on an "Affiliations" table listed with their company from an "Employers" table. Affiliations should belongs_to People and Employers, and Employers and People have_many Affiliations. What would the migration and controller look like?
I'm not entirely sure if this is the same as what you're asking, but using has_many and belongs_to may be a better solution. Using these associations would allow an employee to have many employers and then simply get the most recent one.
Please feel free to correct me if this isn't what you are asking.
For practice I'm writing a shopping website where we have tables User and Item. A user obviously has_many items (when they are added to their basket), but the item, it belongs_to a User, even though many users will have the same item in their basket?
Furthermore, what if I want a list of items a user has added to their basket, but also a list of items they have viewed (for making suggestions based on searches), would it be better to have some 'through' tables: Basket and Viewed?
When you have this many-to-many relationships, you can use the HABTM schema:
Class User...
has_and_belongs_to_many :items
However, most of the time webshops use orderlines to keep up with items that users are purchasing. This means that an 'user' 'has_many' 'orderlines', an 'item' 'has_many' 'orderlines', an 'orderline' 'belongs_to' an 'user' and to an 'item'.
And maybe your orderlines will just be copies of items, and won't have a direct link because you don't want to alter the orderline after they have been processed. It really depends on the focus of your shop which scheme suits your needs.
Try to find some examples on the web and think about how you want to handle items, orders and baskets.
I'm used to separate things that are not the same, even if the relationship is one-to-one. So first of all I would recommend users from baskets (1:1-relationship).
After that a basket contains many items and items can be in multiple baskets (m:n-relationship). Make sure, that maybe a user likes to buy the same item multiple times.
views can be realised as a linking table between users and items: users have many views and items have many views, but one view is always linked to exactly one user and one item.
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.