I am new to rails and this is a very basic question. I am trying to understand the need of foreign key and class_name.
has_many :task, foreign_key: "created_by"
has_many :memberships, class_name: "TaskMembership"
Can anyone explain the need of foreign_key & class_name.
Here is the answer of my question
Suppose you have a User model and Post model.And you have to set an association like User has many post
User Model
has_many :posts
Post Model
belongs_to :user
Now suppose your user is some author so we have to set some meaningful name so instead of user we will use author but have to specify which class it is referring
Post Model
belongs_to :author, class_name: 'User'
Now problem will occur because rails will look for author_id column in posts table .So here foreign key will come into picture.We will have to find user_id
Post Model
belongs_to :author, class_name: 'User', foreign_key: 'user_id'
See more better explanation association
has_many association is used for for one-to-many type relationships in rails. For instance, if you have a model User which can has many profiles, your User to Profile association will be has many.
class User < ActiveRecord::Base
has_many :profiles
end
class Profile < ActiveRecord::Base
belongs_to :user
end
If you have a foreign key different than user_id in profiles table, you explicitly specify foreign_key. Same is the case with class name. If your association name is different than actual model name, you explicitly specify class name after association (as you did for memberships).
Hope it helps.
in your model
class First < ActiveRecord::Base
has_many :seconds
end
class Second < ActiveRecord::Base
belongs_to :first
end
and in your second class table,create first_id column
Related
I have a model called Company, this model is used for both companies and clients, I know this is not the best approach for companies/clients but the software I'm working on was built that way and to change that, it would take months of job.
class Company < ActiveRecord::Base
has_many :companies
belongs_to :company
has_many :orders, dependent: :destroy
end
My other model is Orders, where the companies are linked by the company_id column and the customers by the customer_id column, that is, an order is linked to a company and a customer of that company.
class Order < ActiveRecord::Base
belongs_to :company
end
Currently, I created a method to get the customer but I would like to call a has_one to get the customer from the order, but I am not able to point the customer_id column of the order to the company id column using the rails class_name.
If I understand your question correctly (in the first para you mentioned clients and in the second you are saying customers so I am assuming clients == customers and customer is also a Company object) you are trying to do this:
class Order < ActiveRecord::Base
belongs_to :company
has_one :customer, class_name: 'Company'
end
which seems incorrect as it should be belongs_to:
class Order < ActiveRecord::Base
belongs_to :company
belongs_to :customer, class_name: 'Company'
end
This should work but you can also specify foreign_key:
belongs_to :customer, class_name: 'Company', foreign_key: :customer_id
From the docs: https://api.rubyonrails.org/v7.0.2.4/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_one
has_one specifies a one-to-one association with another class. This method should only be used if the other class contains the foreign key. If the current class contains the foreign key, then you should use belongs_to instead.
Could somebody explain me is my code correct.
I'm trying to get foreign_key option in rails associations.
I have 2 models:
Book and Author
Book db schema:
name
user_id
Author db schema:
name
My models:
class Author < ApplicationRecord
has_many :books, foreign_key: :user_id
end
class Book < ApplicationRecord
belongs_to :author, foreign_key: :user_id
end
Here I don't understand why we should define foreign_key in both models. Is it necessarily?
If you have used the table and column names that Rails expects, then you do not need to explicitly define the foreign_key. In your case, if the foreign key column was named author_id, then you could get by quite simply:
class Author < ApplicationRecord
has_many :books
end
class Book < ApplicationRecord
belongs_to :author
end
However, in your case, the foreign key column is not named according to what Rails expects, so you have needed to explicitly define the foreign key column name. That's fine, but it does make a little more work for you.
In cases where you have explicitly defined the foreign key, you should define it for both associations. Your has_many association will not work without it.
In addition, you should define the inverse association:
class Author < ApplicationRecord
has_many :books, foreign_key: :user_id, inverse_of: :author
end
class Book < ApplicationRecord
belongs_to :author, foreign_key: :user_id, inverse_of: :books
end
Defining the inverse_of can cause ActiveRecord to make fewer queries, and gets rid of a few surprise behaviors. For an explanation of inverse_of, see Exploring the :inverse_of Option on Rails Model Associations by Ryan Stenberg
I am trying to understand the difference between foreign_key and primary_key when defining an association on an ActiveRecord model. Do you need to use both? What is the difference?
class SomeModel < ActiveRecord::Base
has_one :something_else, foreign_key: 'some_id', primary_key: 'some_id'
end
class SomethingElse < ActiveRecord::Base
self.primary_key = 'some_id'
belongs_to :some_model, foreign_key: 'some_id', primary_key: 'some_id'
end
primary_key defines the name of the database field to be used as primary key instead of the default id.
foreign_key defines the name of the database field which keeps references to the primary key field of another model.
See Active Record Associations docs for more.
If you want conceptual understanding on the role of primary and foreign keys, here is one writeup on this topic.
They are on the opposite sides of the association.
Rails takes value of foreign_key and searches model with class_name for records with primary_key of that value
For example:
class PhoneNumber < ActiveRecord::Base
belongs_to :user
end
class User < ActiveRecord::Base
has_one :phone, foreign_key: :this_users_phone_id_field
has_many :others_with_this_number, ->(user){where.not(id: user.id)},
class_name:'User',
foreign_key: :this_users_phone_id_field,
primary_key: :this_users_phone_id_field
end
See more in rails docs
I have two models User and UserRelation. The case is that User has several related users with himself(recommended by him), but he has only one person related_to(person who recommended him).
I would like to return from User object collection of recommended users and user who recommended him. I have written association for returning users collection and it works but I have no idea how should I write has_one association.
I get this error:
ActiveRecord::HasOneThroughCantAssociateThroughCollection: Cannot have a has_one :through association 'User#relation' where the :through association 'User#user_relations' is a collection. Specify a has_one or belongs_to association in the :through option instead
User model:
class User < ActiveRecord::Base
has_many :user_relations
has_many :related_users, through: :user_relations, source: :related_user
has_one :relation, through: :user_relations, source: :user
end
UserRelation model:
class UserRelation < ActiveRecord::Base
belongs_to :user
belongs_to :related_user, class_name: 'User'
end
UserRelation columns:
user_id
related_user_id
My choice would be to put a foreign key in your User table for the possible related_to field.
If the requirement is that it can only be one (or none) then why not?
You still keep the other "user_relations" for all other types. All the time in rails, we map to the same entity in different ways. It's not uncommon at all
I have two models: Person, and Property. In the Person model I have a field which stores the role of the person(tenant, owner, admin, contractor, etc). Since each property will belong to an owner and potentially have one or more tenants, I thought this would be a good opportunity to use the HABTM model relation.
Do I have this right?
Also, how do I reference the attached model? Assuming my join model is named PropertiesPeople, and I wanted to fetch the tenants for a particular property, would that be the following?
#property.people.where(:role => "tenant")
If the same Person can have more than one Property, you should can use HABTM. Something like this:
class Person < ActiveRecord::Base
# in the people table you are storing the 'role' value
has_and_belongs_to_many :properties, join_table: 'people_properties'
end
class Property < ActiveRecord::Base
has_and_belongs_to_many :people, join_table: 'people_properties'
end
You should create the intermidiate table people_properties with the foreign keys, person_id and property_id.
The problem of this approach is that if a Person can be "tenant" in one property and "contractor" in another, for example, you can't store that information. In that case I will suggest using an intermidiate model, like this
class Person < ActiveRecord::Base
has_many :property_people
has_many :properties, through: :property_people
end
class PropertyPerson
# now you store here the 'role' value
belongs_to :person
belongs_to :property
end
class Property < ActiveRecord::Base
has_many :property_people
has_many :people, through: :property_people
end
I don't know for sure if the class names are successfully inferred from the relationships names, in that case you can always indicate class_name or even the foreign_key for the associations. Also you can indicate the table for a model, using self.table_name=