I have a design question.
There are three tables: Users, Posts, and offers.
Offers and Posts are related based on the postid, and users and posts are related on user id.
Now, users will posts something, it could be something to buy or sell. Now users make an offer to buy a post that is being posted. One user can offer to buy multiple posts. So this means one user can have multiple offers for multiple posts. How can i achieve that a user has many offers for different posts. One user can have offer only one posts (meaning there will be only one offer for one post).
Looks simple but am kind a lost, any advice would be appreciated.
I think you are describing a relationship like this:
[Users]1---A---m[Posts]1---B---m[Offers]
[Users]1---C---m[Offers]
The key for relationship A (one-to-many) is userId; the key for relationship B (one-to-many) is postId; and the key for relationship (one-to-many) C is userId.
I think you are missing relationship C in your description. This will require Offers to have two foreign keys; userId and postId. The combination of userId and postId can also be the primary key for Offers because that combination should always be unique.
Related
I am very comfortable with associations but have come across a bit of a unique situation.
I am pulling data from an API. Two models. One is golf clubs, the other golf courses. A club can have multiple courses, and courses belong to a club.
The API already identifies the Club with a unique ID and a unique ID for the club.
How do I set the association so I basically tell it: "Use these pre-existing ID's for your association" rather than generating a unique ID on it's own like it would for a typical association?
Thank you!
Let's say that each Product has a category. I want to ask the Users to select several categories that the user is interested in, and find the Products that have the same category. This is similar to what Quora, Stumbleupon, and Pinterest all do.
What would be the best way to set this database structure in Rails? Should I create 3 tables: User, Product, and Category, and make the relations
User has many Categories & Product has many Categories?
The problem I see with this is doesn't it create, rather than reference, a new instance of Categories to each row of Users and Products?
*extra: What if I wanted subcategories? For example, if the user chose Technology, it could further ask to choose between web dev, mobile dev, hardware, etc.
You could do that kind of 'recommendation' pretty easily.
Something like this should work (N.B.: I did not test this code, but it is right in spirit):
def recommended_products
joins(:categories, :products).where("product_id not in (?)", self.products)
end
Explanation of each bit:
joins(:categories, :products): this does a SQL join of users, products, and categories. This gives you a 'table' where each user-product-category combination is in it's own row.
.where("product_id not in (?)", self.products): adds a SQL where clause to filter out all the rows that have products in the current user's list of products.
The associations are not a problem. They don't create any new instances by themselves, only if you write code that creates new instances yourself.
As for sub categories, I think you'll do better to make that it's own question, as it's easily a whole post in itself.
The mongoid documentation told me that n-n relations should be used with caution
I understand his but don't have an idea how to solve my problem a better way using pure mongoid:
A course has many participants and a participant could participate with many courses. So wouldn't it be faster to store the participant on the course model and do a search over all courses when all courses of a participant are needed?
Your model should be reflective of your use cases.
One way to do this would be to have one model for the courses, one for participants and a 3rd that maps students to courses (with a unique index on course & student to prevent duplicates). This way there is a single model referring to the other 2. This may or may not be ideal based on your access patterns.
I think this is probably a good use case for embedding documents. See the sample syntax on the front page for embeds_many and embedded_in: http://mongoid.org/en/mongoid/
The main downside here is that if you have participants in more than one course, you will have duplicate participants in each of those courses.
Make sure you put an index on the fields you plan to do your lookups for participants with.
I am working on an application that allows students to create a catalog of courses they are taking for a semester. I have created models for user; course; subject and category. Users can have many courses. Each course can have many subjects and categories. The tables for courses, subjects and categories include the following:
Catalog: user_id and course_id
Courses: user_id; coursedetail_id, subject_id and category_id
Coursedetail: name; description
Subject: name; description
Category: name; description
The idea is that an Admin can create a list of courses; subjects and categories and that the user can select the courses they want to add to their catalog. I have seperated courses and coursedetails because I envision that the coursedetails will grow overtime and the courses table will allow me to join the user_id and cousres details to rreport on if necessary.
I attempted to follow Ryan's railscast on Complex Forms thinking that that I should use a complex form and has many relationship to get things working -- but I get an error in the catalog controller - cannot locate catalog_id which I know is in the table. I am now not sure if complex forms is the way to go or I should be looking at another direction to get the appropriate form in place. Any assistance would be appreciated.
I have decided to move ina different direction and feel that the question I posed may have been to broad to get a direct response. for that I apologize. I will in the future make sure questions are specific and backed up by clear code to get the apporpriate feedback and response.
I have a standard devise user model with the usual fields.
This is for a situation where people are either looking for a place to stay, or they have a place to stay. So I have two categories of user that a person can be. These two categories are very distinct (i.e. a person looking for a place to stay will have very different fields to a person who has a place to stay).
So a User has:
User: name, email, password, profile_id
A User can also have a Profile (i.e. they are looking for a house).
Profile: age, sexuality, religion, occupation
That's what I have at the moment. Now I need to change that slightly, so a User can have a profile OR... they can have a House (i.e. they have a house and are looking for more people):
House: price_per_week, address, etc
How best to model this in ActiveRecord? Polymorphic association of some kind?
I've found in general that polymorphic relationships don't work well over time if the objects they are modeling are even mildly different. For your case I'd recommend keeping the two objects separate.
In general, the best way is to consider the way you want to retrieve the data. For example, I'd imagine you want to access both:
#user.house
or
#user.profile
So I'd recommend beginning by setting up relationships (that can be optional) between the users table and both the profiles and houses table. I'd also add a type field that can be used to determine which of the two types the users are.
This allows users to be of either type and allows them to have both a profile and a house.
So both houses and profiles belong_to users, and users have_many (or have_one) houses and profiles