How to handle cardinality 1-to-2 with Associations ?

rubyonrails-talk
1

Hello,

I am new to RoR and am coding a small budgetting app with two tables (see below). As you can see every account has_many transactions and every transaction has 2 accounts. How can I best respresent this using ActiveRecord::Associations ?

create table accounts (   id int not null auto_increment,   name varchar(100) not null default '',   primary key(id) );

create table transactions (     id int not null auto_increment,     month varchar(3) not null,     amount int null,     from_id int not null,     to_id int not null,     constraint fk_from_accounts foreign key (from_id) references accounts(id),     constraint fk_to_accounts foreign key (to_id) references accounts(id),     primary key(id) ) engine=InnoDB;

I see 3 possible solutions but nothing elegant: - create 2 relationship tables (from and to) - use sql - code belongs_to_2 association

I would very much like your advice on this.

Thanks ! Simon

2

Hi Simon,

I think you need to model two relationships.

class Transaction < AR::Base   belongs_to :creditor, :class_name => 'Account', :foreign_key => 'creditor_id' #from_id   belongs_to :debitor, :class_name => 'Account', :foreign_key => 'debitor_id' #to_id end

class Account < AR:Base   has_many :credits, :class_name => 'Transaction', :foreign_key => 'creditor_id' #from_id   has_many :debits, : class_name => 'Transaction', :foreign_key => 'debitor_id' #to_id end

Steve

3

That certainly works. You can also put in a validation for the transaction and the account classes to limit the number of associations between them if you want to treat all associations uniformly.

Michael

4

Thanks you Steve, that is exactly what I was looking for. (I got so close but couldn't figure it out :slight_smile: ).

Simon