I am thinking of creating the following models using 'has_many :through':
class Contract < AR::Base
has_many :clientlines
has_many :codelines
has_many :clients, :through => :clientlines
has_many :codes, :through => :codelines
end
class clientlines < AR::Base
belongs_to :contract
belongs_to :client
end
class Client < AR::Base
has_many :clientlines
has_many :contracts, :through => :clientlines
end
class codeline < AR::Base
belongs_to :contract
belongs_to :code
units_alloc -------**I would like to add this attribute after this intermediate
end has been created?
class Code < AR::Base
has_many :codelines
has_many :contracts, :through => :codelines
end
Do I first create the models with 'rails generate model Contract authnum:string, client_id:integer, st_date:date, end_date:date' for example.
Then fill in all of the associations before the migrations?.
Also, my understanding is that all of the join tables are created automatically by rails when using the has_many :through association. When does that happen?
Lastly, as indicated by the **, can I have this attribute in codelines, and do I create a 'rails generate migration add_units_alloc_to_codelines units_alloc:number' in order to add this attribute to the join table? I was also wondering how I declare the number to be too two decimal places?
If you have the time and inclination could you please comment on my proposed design for my database?
Thanks.
by using has_many :through you use a third model that makes the connection between the other two, so rails doesn't automatically build that model, you build it yourself and reference the foreign keys for the other two models.
don't name your models at their plural, always singular. if you can't name them at singular, you're doing it wrong
The order in which you create your models shouldn't matter too much (rails generates some migrations which you can easily modify later)
That units_alloc attribute, just add it to the model when you create it, simple as that!
for 2 decimals use something like t.decimal :amount, :precision => 6, :scale => 2 in your migration (that example gives you 6 digits and 2 decimals)
Read the Rails Guides, it will really help you get out a lot of trouble
Related
I have a stores application on github. I'm trying to implement counter_cache for two models, 1. Divisions model and 2. Products model. For some reason I'm not sure of the counter cache(divisions_count) doesn't get incremented automatically for the Company model whenever I create a new Division and similarly the products_count doesn't get incremented for the Divisions model when I add a new product to the division.
I'm on rails 3.2.11 and on ruby 1.9.3-p327
My application is only at POC level.
PFB the model structure wrt Company, Division and Product:-
company.rb
class Company < ActiveRecord::Base
attr_accessible :contact_no, :email_id, :fax_no, :name, :website, :divisions_count
has_many :divisions #just added divisions_count to attr_accessible not sure if it helps
has_many :products, :through => :divisions
end
division.rb
class Division < ActiveRecord::Base
attr_accessible :company_id, :name, :products_count
#just added products_count to attr_accessible not sure if it helps
belongs_to :companies, :counter_cache => true
has_many :products
end
product.rb
class Product < ActiveRecord::Base
attr_accessible :division_id, :model, :name, :price
belongs_to :divisions, :counter_cache => true
end
In case you want to refer to the migrations that I've created for the counter cache implementation , you may find them here.
I believe the problem is that you’ve set up belongs_to incorrectly by using the plural name. Switching to singular solves the problem, i.e.
# pseudo diff:
- belongs_to :companies, :counter_cache => true
+ belongs_to :company, :counter_cache => true
- belongs_to :divisions, :counter_cache => true
+ belongs_to :division, :counter_cache => true
When editing models/associations I found it helpful to think of an actual instance. So, it makes sense that the “Windows”-division belongs to “Microsoft”-company, but it makes no sense that it belongs to “Microsoft”-companies. Or just remember belongs_to is always singular and has_many is always plural.
If you need your divisions to belong to multiple companies, you need to use a different association called “has and belongs to many” or HABTM for short (see [1]).
[1] http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
I have a model named Container. This model just has a list of associated Links. So I created tables containers, links and containers_links. Now I want to connect my two models. So I did
class Container < ActiveRecord::Base
has_many :links, :through => :containers_links
end
class Links < ActiveRecord::Base
has_many :containers, :through => :containers_links
end
But I have an error because I don't have reflection with containers_links in my model. I can add something like
has_many :containers_links
But I don't have a model ContainersLinks (and I don't want to create one). What should I do?
You can use has_and_belongs_to_many relationship
class Container < ActiveRecord::Base
has_and_belongs_to_many :links
end
class Links < ActiveRecord::Base
has_and_belongs_to_many :containers
end
But you still need to create join table.
I'm trying to do a basic model association in rails.
Basically I have a List table which stores item_id and user_id.
One user can create multiple "list-items."
Is this the correct way to do it?
Thanks.
class Item < ActiveRecord::Base
has_many :users, :through => :lists
end
class User < ActiveRecord::Base
has_many :items, :through => :lists
end
class List < ActiveRecord::Base
belongs_to :user
belongs_to :item
end
Depending on what you want to reach, your solution is the right one (or not). I see the following cases:
You want to create an n:m association between items and users. So each item could be referenced by many users, and each user references many items. If this is the right context, then your solution is the right one. See the Rails Guides: Associations for more information on that.
An alternative for that situation could be to use the has_and_belongs_to_many Association. The situation is the same, but it does not make sense to talk about lists, there will be no model object for it.
If each users may have many lists, and each list may have many items, your solution would be wrong. This would be no n:m association with list as the join table in between, but two 1:n relations.
The code for the third example would look like that:
class User < ActiveRecord::Base
has_many :items, :through => :lists
has_many :lists
end
class List < ActiveRecord::Base
has_many :items
belongs_to :user
end
class Item < ActiveRecord::Base
belongs_to :list
end
In the first solution, you should add the relations for users to lists and items to list:
class Item < ActiveRecord::Base
has_many :lists
has_many :users, :through => :lists
end
class User < ActiveRecord::Base
has_many :lists
has_many :items, :through => :lists
end
If the "list" entity truly is a pure association/join, that is, it has no inherent attributes of its own, then you can simplify a bit and use has_and_belongs_to_many. Then you don't need a "List" class.
class Item < ActiveRecord::Base
has_and_belongs_to_many :users
end
class User < ActiveRecord::Base
has_and_belongs_to_many :items
end
Rails will look for the references in a "items_users" table, so in your migration, you need to create it a la:
create_table :items_users, :id => false do |t|
t.references :users, :items
end
Many people will tell you to always use has_many :through, but others (like me) will disagree - use the right tool for job.
I guess i have a rather simple question since I'm new to Ruby and even newer to ActiveRecords.
What I want to achieve is a class representation with ActiveRecords (and the corresponding SQL schema) that models the following problem:
There exist categories and subcategories (modeled by parent_id)
Products belong to only one category
Each product can have 0..inf features
Features simply have some data fields and are only referenced by the products
My current schema is shown below in the picture:
Is this schema suitable for ActiveRecords? How would the classes look like? I simply cant figure out how the JoinTable fits into the ActiveRecord structure.
Further, how can i model the link from parent_id->categories.id?
Any help appreciated!
cheers
To model the relationships you described you would do:
models/category.rb
class Category < ActiveRecord::Base
has_many :products
has_many :subcategories, :class_name => "Category", :foreign_key => :parent_id
end
models/product.rb
class Product < ActiveRecord::Base
belongs_to :product
has_many :features, :through => :product_features
has_many :product_features
end
models/feature.rb
class Feature < ActiveRecord::Base
has_many :product_features
has_many :products, :through => :product_features
end
models/productfeature.rb
class ProductFeature < ActiveRecord::Base
belongs_to :product
belongs_to :feature
end
Given this structure then you have the join modelled as a Many-to-Many relation. This is useful since the HABTM style of join is going away in Rails 3.1
To get the information, I often use the console rails console for testing and this would allow you do do
#category = Category.first #get the first category
#category.subcategories #returns an array of categories
The traversal of the links is via the relations that you setup in the models, with the intention that its readable, in the context of using sensible names. The self-joins, as per your question, is also covered in Rails Guides: Associations with a good example. The rest of this guide also details the other relationships.
One other thing to remember is to create your migrations so that the join table is created with the id's which are the foreign keys.
My models would look like this:
class Category < ActiveRecord::Base
has_many :products
end
class Product < ActiveRecord::Base
belongs_to :category
has_many :product_features
has_many :features, :through => :product_features
end
class ProductFeature < ActiveRecord::Base
belongs_to :product
belongs_to :feature
end
class Feature < ActiveRecord::Base
has_many :product_features
has_many :products, :through => :product_features
end
Rails has an association called has_and_belongs_to_many. Rails expects a table with two columns to store the join data. I usually use dual has_many to achieve the same results as it gives you flexibility to add additional information in the join table.
Sample code
product.category
product.category = category1
category.products
category.products << product1
product.features
product.features << feature1
feature.products
feature.products << product1
Here is the API for ActiveRecord::Associations::ClassMethods
There are a lot of examples in there of different relationships and how to construct them. It's worth taking the time to understand how/why you construct these associations.
For the Many-to-many join you will want to look at
has_many ..., :through => ...
has_and_belongs_to_many ...
The docs explain when and why to use each.
I'm pretty new to Rails, and i'm trying to do a polymorphic HABTM relationship. The problem is that I have three models that I want to relate.
The first one is the Event model and then are two kind of attendees: Users and Contacts.
What I want to do is to be able to relate as an attendee both users and contacts. So, what i have right now in my code is:
Event Model
has_and_belongs_to_many :attendees, :polymorphic => true
User Model
has_and_belongs_to_many :events, :as => :attendees
Contact Model
has_and_belongs_to_may :events, :as => :attendees
How the HABTM table migration needs to be? I'm a little confused and i have found no help on that.
Is it going to work?
No, you can't do that, there's no such thing as a polymorphic has_and_belongs_to_many association.
What you can do is create a middle model. It would probably be something like this:
class Subscription < ActiveRecord::Base
belongs_to :attendee, :polymorphic => true
belongs_to :event
end
class Event < ActiveRecord::Base
has_many :subscriptions
end
class User < ActiveRecord::Base
has_many :subscriptions, :as => :attendee
has_many :events, :through => :subscriptions
end
class Contact < ActiveRecord::Base
has_many :subscriptions, :as => :attendee
has_many :events, :through => :subscriptions
end
This way the Subscription model behaves like the link table in a N:N relationship but allows you to have the polymorphic behavior to the Event.
Resolveu parcialmente.
It does solve the problem given the framework that we have at our disposal, but it adds "unnecessary" complexity and code. By creating an intermediary model (which I will call B), and given A -> B -> C being "A has_many B's which has_many C's", we have another AR Model which will load one more AR class implementation into memory once it is loaded, and will instantiate for the sole purpose of reaching C instances. You can always say, if you use the :through association, you don't load the B association, but then you'll be left with an even more obsolete model, which will only be there to see the caravan pass by.
In fact, this might be a feature that is missing from Active Record. I would propose it as a feature to add, since it has been cause of concern for myself (that's how I landed in this post hoping to find a solution :) ).
Cumprimentos