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
Related
Ok so what I have been doing for categories is I have had 2 tables Categories(id, name) and SubCategories(id, name, category_id) that are related through the models. I set inverse_of as you will see below.
My question is when I use to code in PHP years ago we use to have one table "Categories" and it had id, name, parent_id(0 by default) we then used that one table to control the outputs of navigations, breadcrumbs, and other navigational elements.
class CreateCategories < ActiveRecord::Migration[5.1]
def change
create_table :categories do |t|
t.string :name
t.timestamps
end
end
end
class CreateSubCategories < ActiveRecord::Migration[5.1]
def change
create_table :sub_categories do |t|
t.string :name
t.integer :category_id
t.timestamps
end
end
end
class Category < ApplicationRecord
has_many :sub_categories, inverse_of: :category
end
class SubCategory < ApplicationRecord
belongs_to :categories, inverse_of: :sub_category
end
I have been programming in Ruby on Rails now for over 4 years and I have yet to find a real nice "Rubyway" to achieve this. Over the years I have seen examples of using what I am already using with the 2 table method, however this does not seem very intuitive because when the system gets many categories and sub categories like 100's the page load time will be impacted in processing. Is anyone useing or know of a one table method like
class CreateCategories < ActiveRecord::Migration[5.1] def change
create_table :categories do |t|
t.string :name
t.integer :parent_id
t.timestamps
end
end
end
The problem I have always had is the model and how to get the system to realize that a record can belong to a record on the same table. I have been able to achieve it manually but I have not found a way to set it up where formtastic and other gems like rails admin would play nice with it.
Seems that you're looking for a tree structure. acts_as_tree has been around for some time. If you're using PostgreSQL, the ltree extension may be of interest as well (along with the pg_ltree gem).
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'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.
I am in the process of building a multiclient system in ROR. (I am looking at http://guides.rubyonrails.org/association_basics.html#polymorphic-associations)
The structure is that a client has a contract, so when he logs in with his username, password and contract, he will have access to the system.
We have the contract id as a “master key”, which has to be in every table in the system.
class CreateContracts < ActiveRecord::Migration
def change
create_table :contracts do |t|
t.integer :contract_id
end
end
end
(chart of accounts)
class CreateCoas < ActiveRecord::Migration
def change
create_table :coas do |t|
t.integer :account_id
t.string :account_name
end
end
end
class CreateCustGroups < ActiveRecord::Migration
def change
create_table :custgroups do |t|
t.integer :account_id1
t.integer :account_id2
t.integer :account_id3
end
end
end
Q1: How do I define the contract with belongs_to? There has to be a relation in every table in the system to the contract table. Do I have to have a relation to all tables? (I think so)
class Contracts < ActiveRecord::Base
has_and_belongs_to_many :Coas
has_many:xxx
belongs:to
end
Q2: How do I define the association on the custgroup? Here we have a record where I have 3 or more fields that link to the same table (COA).
As Jesper said, it's quite hard to follow what you're trying to achieve, but I'll try to reply to your questions :
Q1 : If you want all your tables to reference a contract, you'll need to add to all those tables a foreign_key such as contract_id
so each create_table call will have the contract_id key
create_table :new_models do |t|
t.belongs_to :contract # this will create a contract_id field
end
you can also add an index on the column
add_index :new_models, :contract_id
then in all you models you'll add the belongs_to association :
class NewModel
...
belongs_to :contract
...
end
so if your Coas & CustGroups needs to reference the contract table, you'll have to change both migrations to include the contract_id key and then the models to add the belongs_to association
If a contract needs to have access to all Coas that references it, then you need to use the has_many association
class Contracts < ActiveRecord::Base
...
has_many :coas
...
end
It doesn't look like you need a has_and_belongs_to_many here, but i might be wrong about that.
if a contract also needs to access to CustGroups, you'll add :
has_many :cust_groups in the Contract model.
Q2 : I really didn't get understand what you want to do. Please explain what is the relation between Coas and Custgroups and I'll try to help you
Another basic Rails question:
I have a database table that needs to contain references to exactly two different records of a specific data type.
Hypothetical example: I'm making a video game database. I have a table for "Companies." I want to have exactly one developer and exactly one publisher for each "Videogame" entry.
I know that if I want to have one company, I can just do something like:
script/generate Videogame company:references
But I need to have both companies. I'd rather not use a join table, as there can only be exactly two of the given data type, and I need them to be distinct.
It seems like the answer should be pretty obvious, but I can't find it anywhere on the Internet.
Just to tidy things up a bit, in your migration you can now also do:
create_table :videogames do |t|
t.belongs_to :developer
t.belongs_to :publisher
end
And since you're calling the keys developer_id and publisher_id, the model should probably be:
belongs_to :developer, :class_name => "Company"
belongs_to :publisher, :class_name => "Company"
It's not a major problem, but I find that as the number of associations with extra arguments get added, the less clear things become, so it's best to stick to the defaults whenever possible.
I have no idea how to do this with script/generate.
The underlying idea is easier to show without using script/generate anyway. You want two fields in your videogames table/model that hold the foreign keys to the companies table/model.
I'll show you what I think the code would look like, but I haven't tested it, so I could be wrong.
Your migration file has:
create_table :videogames do |t|
# all your other fields
t.int :developer_id
t.int :publisher_id
end
Then in your model:
belongs_to :developer, class_name: "Company", foreign_key: "developer_id"
belongs_to :publisher, class_name: "Company", foreign_key: "publisher_id"
You also mention wanting the two companies to be distinct, which you could handle in a validation in the model that checks that developer_id != publisher_id.
If there are any methods or validation you want specific to a certain company type, you could sub class the company model. This employs a technique called single table inheritance. For more information check out this article: http://wiki.rubyonrails.org/rails/pages/singletableinheritance
You would then have:
#db/migrate/###_create_companies
class CreateCompanies < ActiveRecord::Migration
def self.up
create_table :companies do |t|
t.string :type # required so rails know what type of company a record is
t.timestamps
end
end
def self.down
drop_table :companies
end
end
#db/migrate/###_create_videogames
class CreateVideogames < ActiveRecord::Migration
create_table :videogames do |t|
t.belongs_to :developer
t.belongs_to :publisher
end
def self.down
drop_table :videogames
end
end
#app/models/company.rb
class Company < ActiveRecord::Base
has_many :videogames
common validations and methods
end
#app/models/developer.rb
class Developer < Company
developer specific code
end
#app/models/publisher.rb
class Publisher < Company
publisher specific code
end
#app/models/videogame.rb
class Videogame < ActiveRecord::Base
belongs_to :developer, :publisher
end
As a result, you would have Company, Developer and Publisher models to use.
Company.find(:all)
Developer.find(:all)
Publisher.find(:all)