I am trying to link two tables to each other
class Musers < ActiveRecord::Base
# Email
# sid (student_id:integer)
# isyk: boolean
belongs_to :user, :foreign_key => "smail"
end
class Users < ActiveRecord::Base
belongs_to :muser, :foreign_key => "email"
end
But,
#user = Users.first
#user.muser returns nil
By saying :foreign_key => "smail" you are telling rails that the Muser column smail points to the User model's foreign key. This is most likely not the case.
Assuming that the primary key of the User models is called id, you should a user_id field to Muser, and change belongs_to :user, :foreign_key => "smail" into:
belongs_to :user
On the User model you can define the reverse relation using:
has_one :muser
Also, to follow rails model naming conventions, you should rename Users to User and Musers to Muser.
You should read more about belongs_to and has_one.
If, on the other hand, the User model in fact uses email for it's primary key, I would strongly advise you to change that and add an auto-incrementing primary key instead. As a rule of thumb, the primary key should be chosen such that it never changes. If it does change, all foreign keys pointing to that primary key will have to change as well.
You should only use a non auto-incrementing primary key if you have a specific reason for doing so.
More information on choosing a primary key: How to choose my primary key?
Well you can't just tell rails the type of association, you actually have to set the association to an instance of that class. For example, making a new muser will not automatically assign a user as the belongs_to. You could do something like
u = User.new
u.muser = Muser.first
u.save
However, I'm not sure what you are trying to accomplish with a belongs_to - belongs_to relationship, but you should know that you have to do more than just tell rails it exists.
Related
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 Ruby 1.8.7 and Rails 2.3.8 and I have the following root resource:
class PointOfInterest < ActiveRecord::Base
set_primary_key "Id"
set_table_name "POI"
has_many :attributes, :foreign_key => 'POIId'
end
The point of interest can have several attributes:
class Attribute < ActiveRecord::Base
set_primary_key "Id"
set_table_name "Attribute"
belongs_to :point_of_interest, :foreign_key => 'POIId'
has_one :multimedia, :foreign_key => 'Id', :primary_key => 'Value'
end
The attribute class may have media associated with it:
class Multimedia < ActiveRecord::Base
set_primary_key "Id"
set_table_name "Multimedia"
end
I am trying to insert a point of interest in my database like so:
poi = PointOfInterest.new
attr = poi.attributes.new
attr.SomeAttribute = 1
attr.build_multimedia(:content => 'test')
poi.save
This is properly persisting both the root (PointOfInterest) and the Multimedia record. The Attribute, however, is not being properly persisted. While the foreign key to the point of interest is properly set (POIId), the foreign key to the Multimedia record remains null.
Any clue as to why this is very much appreciated!
Thanks!
Your relationship / foreign key are set at cross purposes.
If Attribute has_one Multimedia, then you need the Multimedia model to declare belongs_to :attribute, and the Multimedia table to contain a foreign key for Attribute.
I'm guessing that you can't change your database schema (otherwise, I'd have to ask why you're using non-Rails-standard table and key names); in which case you want to switch the sense of the relation. Make Attribute belongs_to :multimedia, and Multimedia has_one :attribute. Then the Multimedia FK in the Attribute table is pointing in the right direction.
I have a model called "EmployeeRecord" and it has a field called username, There is another model called "Employee" with a field called username.
I want to create an association between the 2 models so when I do:
record = EmployeeRecord.find(1)
record.employee // returns Employee instance
I was thinking I'd just need to do this, but apparently it doesn't work:
class EmployeeRecord < ActiveRecord::Base
has_one :employee, :foreign_key: username
end
Assume I can't add an employee_id field to EmployeeRecord. I scoured the Rails tutorials.. and recall wanting to know how to do this months ago.. but those dang Rails tutorials glided over this.. I remember.. it made me very very angry hehe
Any idea?
You'll also need to specify the primary_key used for the association, or else it's defaulted to "id". Your statement actually says "Search for a field username in table employee that is equal to my id field". What you actually want is "Search for a field username in table employee that is equal to my username field"
This should do the trick :
class EmployeeRecord < ActiveRecord::Base
has_one :employee, :foreign_key => username, :primary_key => :username
end
But hey... Why don't you use ids?
Add the foreign key option in the belongs_to method in your Employee model.
class Employee < ActiveRecord::Base
belongs_to :employee_record, foreign_key: username
end
The has_one or has_many is the parent, so it doesn't store the foreign key value/column. That's what the child does that has the belong_to side of the relationship.
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.