I'm interested in creating and updating a row in table without a primary key. My table has 3 columns - person_id, year and salary. I understand that I should use has_and_belongs_to but I'm having problems understanding how to implement my create and update methods and my form.html file. Can anyone help explain this to me, perhaps with a simple example of how to do it?
has_and_belongs_to_many example
# category model
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# user model
class User < ACtiveRecord::Base
has_and_belongs_to_many :categories
end
join table look like
class CreateCategoriesUsersJoinTable < ACtiveRecord::Migration
def change
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
end
end
now you can accessing your information
$ User.categories
$ Category.users
$ user = User.first
$ user.categories
$ category = Category.first
$ category.users
Add a primary key, and ignore it. You can add a unique index on (person_id, year) to simulate a PK constraint, but ActiveRecord heavily relies on having ids for its instances.
Related
Currently I am having issues with the belongs_to relationship between my Page model and my Companies model.
I receive the following error message when I am on the Page show.erb.html and I am accessing #page.company.id: undefined method `id' for nil:NilClass
This error message with and without the attr_accessor method in the companies model.
Page Model:
class Page < ActiveRecord::Base
belongs_to :company
end
Company Model:
class Company < ActiveRecord::Base
attr_accessor :id, :name
has_many :pages
end
Companies Migration:
class CreateCompanies < ActiveRecord::Migration
def change
create_table :companies do |t|
t.string :name
t.timestamps
end
end
end
Pages migration:
class CreatePages < ActiveRecord::Migration
def change
create_table :pages do |t|
t.string :name
t.references :companies, index: true
t.timestamps
end
end
end
As a page has a single company, your page migration should use:
t.references :company, index: true
(singular) instead of :companies (plural). This should allow rails to populate it properly.
Associations
As Rails is designed to run on a relational database stack, you have to appreciate the role of this structure plays in defining & creating the ActiveRecord associations in your models.
To give you some ideas - ActiveRecord is actually something known as an "ORM" (Object Relationship Mapper) - meaning that it will manage the relationships between "objects" in your app. As Rails, by virtue of being built on top of Ruby, is object orientated, ActiveRecord plays an integral role in managing the relationships between your objects
The problem you're referring to is how to construct these relationships correctly:
Models
The answer to your question is:
#app/models/page.rb
Class Page < ActiveRecord::Base
#fields id | company_id | other | attributes | created_at | updated_at
belongs_to :company
end
#app/models/company.rb
Class Company < ActiveRecord::Base
#fields id | title | other | page | attributes | created_at | updated_at
has_many :pages
end
To demonstrate what the tables should look like, you can use this Rails documentation:
This means that in reference to your question, you can either use Martin's answer, or just add the company_id column manually (we do that):
class CreatePages < ActiveRecord::Migration
def change
create_table :pages do |t|
t.string :name
t.string :company_id
t.timestamps
end
end
end
Datatables
Some other information information you should know:
Rails builds your associations with foreign_keys - whenever you create a "relationship" in your Models, all it does it trigger ActiveRecord to identify the foreign_key for the association:
Finally, you also need to appreciate that the identification of "records" in a datatable is determined by the primary key it has. Every time you load an object in Rails, it's actually looking into the database for the corresponding id it has
This means if you wish to identify records in Rails, you need to make sure they have a primary key in the database
In the below example, do I have to create employee_id in the Office model, or is it created automatically by db:migrate?
class Employee < ActiveRecord::Base
has_one :office
end
class Office < ActiveRecord::Base
belongs_to :employee # foreign key - employee_id
end
Feels like I'm missing something fundamental. I'm trying to get a basic one to many relationship working, where I can use a drop-down select of objects from the one side. Are there any good basic tuts explaining how this works?
I had to create _ids in all the models where I wanted this to work, but it doesn't seem right from examples I've looked at.
two steps.
firstly, you have to create an employee_id field in the office table in the migration file. you will have something like that :
class CreateOffices < ActiveRecord::Migration
def change
create_table :offices do |t|
t.string :name
t.integer :employee_id
t.timestamps
end
end
end
secondly, you have to define the association in the model. by convention, if you name the foreign_key field employee_id, you don't have to specify the name of it in the model.
class Office < ActiveRecord::Base
belongs_to :employee
end
should be enough.
Associations in ActiveRecord comprise two parts. Hooking together the model objects (like you've done) and setting up the database. So you'll need to define the association in your migration like so:
def change
create_table :offices do |t|
# Other migrations
t.references :employee
end
end
Alternatively you can do t.integer :employee_id which will achieve the same end too.
I was under the impression that with Rails you're not supposed to define any dependencies in the database, but rather just use your has_many and belongs_to stuff to define relationships. However, I'm going through the rails guide, and it has the following.
class CreateComments < ActiveRecord::Migration
def change
create_table :comments do |t|
t.string :commenter
t.text :body
t.references :post
t.timestamps
end
add_index :comments, :post_id
end
end
I thought this wasn't okay...? I'm trying to do something like a comment field that creates a new instance each time you call the show method, but I think without these "references" and "add_index," it's not storing the post_id in the comment row.
All this migration does is create post_id and tells the database that it should index this column (improves performance)
t.references :post is basically the same as t.integer :post_id so, yes, it is storing the post_id in the comment. You'll still need to define your relationships in your models.
You are actually wrong on the philosophy.
Rails magic is good, only when backed at the DB level by actual foreign keys.
The docs clearly state this
Rails magic comes in, when you have correctly named your foreign keys, so that it can use the convention to figure out the associations.
What's wrong with expressing relationships within the ORM, that's where it's supposed to be done. I believe you are getting mixed up between db vendor specifics such as foreign key constraints and relationships.
class Comment < ActiveRecord::Base
attr_accessible :post, :post_id
belongs_to :post
end
class Post < ActiveRecord::Base
has_many :comments
end
class CommentsController < ApplicationController
def create
#comment = Comment.create(params[:comment]) # where params[:comment] = {post_id: 1, message: ''}
#post = comment.post
respond_with(#comment)
end
end
I've been racking my head around a rails issue for a while and wanted to verify my findings. I was trying to get the Has_and_belongs_to_many relationship working, but couldn't connect my two classes, auctionItem and category. First of all, here was my migration file and the two classes before solving the issue:
Migration file:
class AuctionItemsCategories < ActiveRecord::Migration
def up
create_table 'auction_items_categories', :id=>false do |t|
t.reference :auctionItem_id
t.references :category_id
end
end
def down
drop_table 'auction_items_categories'
end
end
Category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :auction_items
end
auction_item.rb
class AuctionItem < ActiveRecord::Base
has_and_belongs_to_many :categories
end
After creating an instance of AuctionItem, I tried
auction_item = AuctionItem.last
auction_item.categories
...and got the following error:
NoMethodError: undefined method `categories' for #<AuctionItem:0x0000010521b870>
After some research, I found adding the specific class to the has_and_belongs_to_many helped:
Category Model
has_and_belongs_to_many :auction_items , :class_name => 'AuctionItem'
auction_item Model
has_and_belongs_to_many :categories , :class_name => 'Category'
This solved that issue and I was able to access the categories table. I went on to try to append a category to the auction item:
auction_item.categories << category
I then got received the following error:
SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2
SQLite3::SQLException: no such column: auction_items_categories.auction_item_id: SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: auction_items_categories.auction_item_id: SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2
If you notice, the query is trying to get auction_item.id rather than AuctionItem.id. To get the connection to work, I had to change my migration file to the following:
class AuctionItemsCategories < ActiveRecord::Migration
def up
create_table 'auction_items_categories', :id=>false do |t|
t.integer :auction_item_id
t.integer :category_id
end
end
def down
drop_table 'auction_items_categories'
end
end
So long story short/TL DR version: For me, it seems that when naming your class with multiple words and using camel case, rails does not singularize your pluralized class name back to it's original state if it has an underscore. So for example, my class name was AuctionItem which became auction_items for the model. Rather than search for auctionitem.id, the sql call that was looked for was auction_item.id, which is the singularize version of auction_items. Why didn't it search for auctionitem.id? In the future, when I am making association tables with multi word classes, do I use the singular underscore id version of the model name?
Your original migration was incorrect. It should have been as follows:
class AuctionItemsCategories < ActiveRecord::Migration
def up
create_table 'auction_items_categories', :id => false do |t|
t.references :auction_item
t.references :category
end
end
def down
drop_table 'auction_items_categories'
end
end
You should specify the symbolized model name when using references.
Imagine I have two models
Film
-name
-description
-duration
-year_made
-rating
-actors
Actor
-name
-d_o_b
-biography
-films
Actors are nested in a Film and vice versa.
How do I represent this relationship in my Ruby models? Realistically I would have a third table mapping actor_id with film_id.
Whilst adding details to a film I would like to be able to create an actor on the fly(if an actor does not exist create a new one with the name supplied)
Thank you in advance.
ADDITION:
Just found a link to a similar question.
You're looking at a Has and Belongs to Many (HABTM) relationship between the two tables.
Read about HABTM relationship in the Rails guides here: http://edgeguides.rubyonrails.org/association_basics.html#has_and_belongs_to_many-association-reference
First you'll need to generate a migration which will look something like this:
class AddActorFilmTable < ActiveRecord::Migration
def self.up
create_table :actors_films, :id => false do |t|
t.integer :actor_id, :null => :false
t.integer :film_id, :null => :false
end
add_index :actors_films, [:actor_id, :film_id], :unique => true
end
def self.down
drop_table :actors_films
end
end
and then specify in your models:
class Actor < ActiveRecord::Base
has_and_belongs_to_many :films
end
class Film < ActiveRecord::Base
has_and_belongs_to_many :actors
end
This will allow you to use all of the additional Rails methods for this type of relationship. To use this in a form, you could follow RailsCast 17: HABTM Checkboxes - though it's old, it should still apply. Alternatively, you can use a gem like Simple Form to easily generate the associations for you like so:
form_for #actor do |f|
f.collection_check_boxes :film_ids, Film.all, :id, :name
end