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Questions tagged [associations]

Associations typically refer to relationships between models in ORMs such as ActiveRecord.

Associations typically refer to relationships between models in ORMs such as ActiveRecord.

Typically associations are classified based on the number of associated models:

  • one to many: one of the models stores the ID of the other
  • one to one: models (which 'belong to') each store the ID of the one associated model (which 'has many')
  • many to many: the models are linked via a join table in the database which may (called sometimes 'has many through') or may not (called 'has and belongs to many') be a model itself
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What is the difference between association, aggregation and composition?

What is the difference between association, aggregation, and composition? Please explain in terms of implementation.
None
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What is causing this ActiveRecord::ReadOnlyRecord error?

This follows this prior question, which was answered. I actually discovered I could remove a join from that query, so now the working query is start_cards = DeckCard.find :all, :joins => [:card], :conditions => ["deck_cards.deck_id = ? and…
user26270
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Rails :dependent => :destroy VS :dependent => :delete_all

In rails guides it's described like this: Objects will be in addition destroyed if they’re associated with :dependent => :destroy, and deleted if they’re associated with :dependent => :delete_all Right, cool. But what's the difference between…
Sergey
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What is the difference between Unidirectional and Bidirectional JPA and Hibernate associations?

What is the difference between Unidirectional and Bidirectional associations? Since the table generated in the db are all the same,so the only difference I found is that each side of the bidiretional assocations will have a refer to the other,and…
hguser
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MongoDB Many-to-Many Association

How would you do a many-to-many association with MongoDB? For example; let's say you have a Users table and a Roles table. Users have many roles, and roles have many users. In SQL land you would create a UserRoles table. Users: Id …
Josh Close
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Rails: Using build with a has_one association in rails

In this example, I create a user with no profile, then later on create a profile for that user. I tried using build with a has_one association but that blew up. The only way I see this working is using has_many. The user is supposed to only have at…
espinet
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Rails migration: t.references with alternative name?

So I have a create_table like this for Courses at a School: create_table :courses do |t| t.string :name t.references :course t.timestamps end but I want it to reference two other courses like: has_many :transferrable_as # A Course has_many…
themirror
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Build vs new in Rails 3

In the Rails 3 docs, the build method for associations is described as being the same as the new method, but with the automatic assignment of the foreign key. Straight from the docs: Firm#clients.build (similar to Client.new("firm_id" => id)) I've…
ClosureCowboy
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How to create has_and_belongs_to_many associations in Factory girl

Given the following class User < ActiveRecord::Base has_and_belongs_to_many :companies end class Company < ActiveRecord::Base has_and_belongs_to_many :users end how do you define factories for companies and users including the bidirectional…
opsb
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Rails association with multiple foreign keys

I want to be able to use two columns on one table to define a relationship. So using a task app as an example. Attempt 1: class User < ActiveRecord::Base has_many :tasks end class Task < ActiveRecord::Base belongs_to :owner, class_name:…
JonathanSimmons
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Rails - Best-Practice: How to create dependent has_one relations

What's the best practice to create has_one relations? For example, if I have a user model, and it must have a profile... How could I accomplish that? One solution would be: # user.rb class User << ActiveRecord::Base after_create…
BvuRVKyUVlViVIc7
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Rails has_one :through association

Rails has a has_one :through association that helps set up a one-to-one association with a third model by going through a second model. What is the real use of that besides making a shortcut association, that would otherwise be an extra step…
Anurag
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What is the best way to implement Polymorphic Association in SQL Server?

I have tons of instances where I need to implement some sort of Polymorphic Association in my database. I always waste tons of time thinking through all the options all over again. Here are the 3 I can think of. I'm hoping there is a best practice…
Mark
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Rails: ActiveRecord query based on association value

I have 2 models. Report and Server that have a belongs_to and has_many relationship. I created an accessor method using delegate that allows a Report to find its associated Server.company_id. Now, I want to run a query on Report that allows me to…
user2158382
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add associations to exisiting models

I'm wondering how I can add associations to my models. Suppose, I generate two models rails generate model User rails generate model Car Now I want to add an associations so that the models acquire the form class User < ActiveRecord::Base …
Andrew
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