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/
Related
I want to create an application for enabling the users to book the computers in a laboratory if they want to use it from a specific time to time.The users can book the computers for their next 15 days. So,how should I design the database for this application.
Start by defining what your entities and attibutes will be. It's better if you can do a Conceptual Design first.
Than you design it logically.
For example, your entities might be:
USERS, COMPUTERS, RESERVATIONS.
Your attributes might be:
USERS (SomeUniquePersonalIDnumber, Name, Surname, Email*, PhoneNumber*)
PrimaryKey in bold. With asterisk *Optional
COMPUTERS (UniqueComputerSerialNumber, NumberOfComputerInLab)
RESERVATIONS (AutoincrementNumber, UserPK, ComputerPK, DateOfReservation, TimeFrom, TimeTill)
PrimaryKey composed of the three attibutes making it unique. Same user might reserve the same computer over time but the AutoincrementNumber field will make the composite PK unique.
RESERVATIONS(UserPK) referencing USER(SomeUniquePersonalIDnumber)
RESERVATIONS(ComputerPK) referencing COMPUTERS(UniqueComputerSerialNumber)
Define what type of fields will those attributes be
(Integer/Varchar/...) based on the querying language you will want
to use.
Translate all of above into commands to create the database, the
tables etc.
Just pick a piece of paper and start with normalization https://en.wikipedia.org/wiki/Database_normalization.
If I look for entities in your question I see the following entities:
user
computer
booking
Then you need to figure out what properties belong to those entities and the relation between them. You can create an ERD if you like and start creating Ruby on Rails models.
In Ruby on Rails if you generate a model the model has a created_at datetime field by default. I think you can use that to figure out the specific time and check if 15 days are past since this booking is created.
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.
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.
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
I have an app with an Account model. Each Account belongs_to a sport, which I would usually have as a Sport model and in the DB. But as this isn't really something that will change and is not administered by the end users I thought that it might be better to put it as an integer column in the Account model and map to a hash using a class variable.
However, I need each sport to have many player_positions (which are specific to each sport). So I thought maybe I could do something like:
##player_positions = {:rugby => [position_1, ..., ...]}
Is this good practice for static data like this or should I stick to putting it in the DB as it is relational?
I also thought maybe I could use a yaml file but not sure how I could set that up.
I would stick in the database because you are relating more than one thing to the sport (positions in addition to Account). It makes life easier if you want to use belongs_to and it will allow you to easily use SQL for counts and such (e.g., you can group accounts by sport and get a count of accounts by sport).
I have used an enumerated list of constants in some cases as opposed to tables. For example, I had a simple app with a user type of ADMIN, EDITOR and READER. Rather than put that in a roles table in the DB, I just made those constants on the User class and added a string column to the users table for role.
You don't have to use the database. http://railscasts.com/episodes/189-embedded-association might give you ideas.