What is the difference between a belongs_to and a has_one?
Reading the Ruby on Rails guide hasn't helped me.
They essentially do the same thing, the only difference is what side of the relationship you are on. If a User has a Profile, then in the User class you'd have has_one :profile and in the Profile class you'd have belongs_to :user. To determine who "has" the other object, look at where the foreign key is. We can say that a User "has" a Profile because the profiles table has a user_id column. If there was a column called profile_id on the users table, however, we would say that a Profile has a User, and the belongs_to/has_one locations would be swapped.
here is a more detailed explanation.
It's about where the foreign key sits.
class Foo < AR:Base
end
If foo belongs_to :bar, then the foos table has a bar_id column
If foo has_one :bar, then the bars table has a foo_id column
On the conceptual level, if your class A has a has_one relationship with class B then class A is the parent of class B hence your class B will have a belongs_to relationship with class A since it is the child of class A.
Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class declaring the belongs_to relationship.
class User < ActiveRecord::Base
# I reference an account.
belongs_to :account
end
class Account < ActiveRecord::Base
# One user references me.
has_one :user
end
The tables for these classes could look something like:
CREATE TABLE users (
id int(11) NOT NULL auto_increment,
account_id int(11) default NULL,
name varchar default NULL,
PRIMARY KEY (id)
)
CREATE TABLE accounts (
id int(11) NOT NULL auto_increment,
name varchar default NULL,
PRIMARY KEY (id)
)
has_one and belongs_to generally are same in a sense that they point to the other related model. belongs_to make sure that this model has the foreign_key defined.
has_one makes sure that the other model has_foreign key defined.
To be more specific, there are two sides of relationship, one is the Owner and other is Belongings. If only has_one is defined we can get its Belongings but cannot get the Owner from the belongings. To trace the Owner we need to define the belongs_to as well in the belonging model.
One additional thing that I want to add is, suppose we have the following models association.
class Author < ApplicationRecord
has_many :books
end
If we only write the above association, then we can get all books of a particular author with
#books = #author.books
but, for a particular book, we can't get the corresponding author with
#author = #book.author
To make the above code work we need to add an association to the Book model as well, like this
class Book < ApplicationRecord
belongs_to :author
end
This will add method 'author' to the Book model. For mode details see guides
has_one
This method should only be used if the other class contains the foreign key.
belongs_to
This method should only be used if the current class contains the foreign key.
From a simplicity standpoint, belongs_to is better than has_one because in has_one, you would have to add the following constraints to the model and table that has the foreign key to enforce the has_one relationship:
validates :foreign_key, presence: true, uniqueness: true
add a database unique index on the foreign key.
Related
In a Rails 4 application, I have an STI model that stores metadata in a jsonb column.
Base Class:
class Post < ActiveRecord::Base
...
end
Subclass:
class JobPost < Post
# has a jsonb column for metadata
end
One of the data attributes in the metadata column of a JobPost is a foreign_key reference to another table (company_id). I'd like to add a belongs_to :company reference in the JobPost model. It seems like this should be possible by doing something like
class JobPost < Post
belongs_to :company do
Company.find_by_id self.metadata['company_id']
end
end
but that doesn't appear to work. Help?
Note: I am not necessarily intent on using belongs_to rather than writing needed methods like def company by hand, but I do need a way to eager load companies when listing job posts. If there's a way to do that eager loading without a belongs_to relation I'm all ears.
Update1
I have also tried the following, which doesn't appear to work either:
class JobPost < Post
belongs_to :company, foreign_key: "(posts.metadata->>'company_id')::integer".to_sym
end
Update2
To be more clear about my intentions and need:
1) A JobPost belongs_to a Company, but a Post (and other subclasses of Post) does not. I'd prefer not to jankily add the company_id column to the posts table when it won't be used by the other subclasses.
2) A JobPost could justify having it's own table (perhaps the relationship with a Company is enough to justify it). There are reasons why this wouldn't be ideal, but if that's the only answer I'm open to it. I'd, however, like a more definitive "what you're trying to do can't be done" response before going down this road, though.
The primary question is whether you can customize belongs_to so that it uses the metadata column rather than expecting the foreign key to be a column in the table.
The secondary question is whether you can eager load companies alongside job posts without having that belongs_to relation set up.
EDIT
UPD 2
You need to add "company_id" column to the base class of your STI table. If JobPost inherits from Post, and it should have "company_id" then add the "company_id" column to Post (base table).
Remember STI stands for "Single Table Inheritance" so there is only one table on database schema level. Imagine a column of a Post table, where few data records are the foreign key entries for Companies with company_id and what about the other records of this column with non JobPost subclass types, are they null/empty? Hence the relationship is defined with parent STI table and subclass inherits these relations. Additional type column in STI defines the subclass type.
Check here
You may need to dig further on Polymorphic classes instead of STI if both JobPost and Post have relationship with Company, else create two separate model, as they tend do have some unique relationships and column fields.
UPD
Based on updated ask
app/model/company.rb
class Company < ActiveRecord::Base
has_many :posts
delegate :jobposts, to: :posts
end
app/model/post.rb
class Post < ActiveRecord::Base
belongs_to :company
self.inheritance_column = :ptype
scope :job_posts, -> { where(ptype: 'JobPost') }
def self.ptype
%w(JobPost)
end
end
app/models/jobpost.rb
class JobPost < Post; end
Create a company
company = Company.create!(company_params)
Create some posts and add them to the company
company.posts << JobPost.new(jobpost_params)
To fetch jobpost by company relationship
company.job_posts
In case you are storing company_id in jsonb in any which column, just format your jobpost_params hash input accordingly and it should do the deed for you
OLD ASK
To find by primary key
Company.find(id)
In your case, id is self.metadata['company_id']
To find by other keys
Company.find_by(key: value)
Company.find_by_id is no more recommended
Please remove do and end after belongs_to in your model, instead in your controller you can write:
Jobpost.all.each do |x|
# your do
end
regarding foreign key, as rails is convention over configuration, it by default includes company_id reference to Jobpost which you can change in your Company.rb model
I have next structure of models in my app:
class Company
has_many :employees
end
class Employee
belongs_to :company
end
Is there a way to make it possible for employees to have unique ids (default primary keys) depending on belongs_to Company association?
These should return different Employee models:
/companies/1/employees/1
/companies/2/employees/1
Thanks!
Try the sequenced gem, it does exactly what you're asking. There is one consideration to keep in mind though.
Your requirement deprives Employee's id field of uniqueness which it needs to be a primary key. Therefore you'd either need to have a composite key in Employee, namely [:company_id, :employee_id] or use the Employee's acts_as_sequenced field not as the primary key but more like a slug.
Just in case you care to explore the composite key approach, there is composite_primary_key gem which aims to support ActiveRecord associations on top of composite keys. I haven't tried it myself though.
According to its docs, your associations could look something like this:
class Company < ActiveRecord::Base
has_many :employees, :foreign_key => [:company_id, :employee_id]
end
class Employee < ActiveRecord::Base
self.primary_keys = :user_id, :employee_id
belongs_to :company
end
But its quite likely this is an overkill approach for your goal.
I got a trouble in my rails project to achieve foreign key concepts, I had gone through http://guides.rubyonrails.org/association_basics.html and few of tutorials and came to know that this can be achieved by association and I had tried using the same but yet to succeed.
Actually I am new to Rails and I continue running into the same path. Any guidance much appreciated...
As per my scenario , I am having two models User and Region
In User model there are three fields user_id, user_name, user_region_id (foreign key)
In Region model => region_id(primary key), region name
I want to display In user_id user_name region_name (using foreign key) on index page of of user
so how an achieve this ??Please help me out
your help will be greatly appreciated
thanks in advance for your help
Conventionally you should not prefix your column names with user_ and region_, because they are already present in the table named like that. Removing the prefix you can achieve your desired result using following code in your models:
class User < ActiveRecord::Base
belongs_to :region
end
class Region < ActiveRecord::Base
has_many :users
end
I think the easiest way to remember association in rails is that the model which has the foreign key should have the belongs_to declaration. So in your setup, since a Region has many users, it would make sense to have the foreign key in the users table.
Assuming that the foreign key in the users table is called region_id, you should declare the following association in the User model.
class User < ActiveRecord::Base
belongs_to :region
end
So this works because rails assumes that the foreign key is under a column called region_id. AND the primary key is under a column called id in the regions table.
On the side of the association, that's when you decide if it's a has_one or has_many. Since it has already been decided that it should be has_many, it's as easy as just declaring that association in the Region model
class Region < ActiveRecord::Base
has_many :users
end
If you want to keep using region_id as the primary key for the regions table, change the association in the user model to
class User < ActiveRecord::Base
belongs_to :region, primary_key: :region_id
end
AND make sure to declare it in the region model so that doing region.users will work
class Region < ActiveRecord::Base
self.primary_key = 'region_id'
end
So I have a DB relationship where a User has a PreferenceList. I have set up a preference_list_id in the database table, and I have set up a belongs_to :user relationship with PreferenceList and a has_one :preference_list relationship with User.
Here's the weird part. If I do user.preference_list = preference_list, I can access user.preference_list and it will give me the correct instance of preference_list. However, if I do user.preference_list_id (a valid, column, I've checked), it gives me nothing even after I have added preference_list to user.preference_list. I need the id to be updated for various database operations. What am I doing wrong?
The foreign key goes on the table for the class declaring the belongs_to association. So if you have
class PreferenceList < ActiveRecord::Base
belongs_to :user
end
class User < ActiveRecord::Base
has_one :preference_list
end
table preferences_lists should have user_id columns. Rails will handle this columns properly. You can add preference_list_id column to users table as well. But this column has no special meaning, Rails will do nothing with it by default. Try to change belongs_to/has_one sides if you really need to operate preference_list_id. Or use user.preference_list.id
I am using an existing database with a rails app.
I can't change the table or column names.
Lets say table one is "invoices" and table 2 is "orders"
they both have a primary key that is called the same thing, lets say "order_id"
invoices can find its order by looking at the primary key "order_id" in the orders table.
Vice versa for an order finding its invoice.
Invoices can have but not always have one order (you might be invoiced for something besides an order, like a "work_order" which would be found by looking for the "order_id" in the primary key position of the "work_orders" table. So invoices might have a work_order or an order.
orders always has an invoice
work_orders always has an invoice
Im trying to figure out the classes in the models.
Do you set the primary and foreign keys to the same thing? What about belongs_to? The way this DB is set up, nothing really belongs to anything, they just reference each other by this same value "order_id". Would it be like this?
class Invoice < ActiveRecord::Base
set_primary_key(:order_id)
set_foreign_key(:order_id)
end
-- snip --
class Order < ActiveRecord::Base
set_primary_key(:order_id)
set_foreign_key(:order_id)
end
... And the same for a work order.
class WorkOrder < ActiveRecord::Base
set_primary_key(:order_id)
set_foreign_key(:order_id)
end
Is this correct? It seems to trashy of a way to do it but this DB is terrible.
What about all the belongs_to stuff?
Let me know if I have left anything out.
Thanks!
I believe the answer could be:
class Order < ActiveRecord::Base
set_primary_key(:order_id)
belongs_to :invoice, :foreign_key => :order_id
end
class WorkOrder < ActiveRecord::Base
set_primary_key(:order_id)
belongs_to :invoice, :foreign_key => :order_id
end
class Invoice < ActiveRecord::Base
set_primary_key(:order_id)
has_one :work_order
has_one :order
end
Although I'm not really sure your primary key can also be a foreign key (I'm new to Rails too)