Referencing a table twice in rails/postgres - ruby-on-rails

Question

I am using PostgreSQL as my db in rails. I have a User model and a Product model. I am trying to make a Transaction model where I want to reference user twice, once as buyer and once as seller. I generated the models as suggested in this post (answer by toasterlovin)
Write a migration with reference to a model twice
However it gives me an error when I use PostgreSQL (with SQLite it worked fine). I get the following error. What can I do to resolve this?
{StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedTable: ERROR: relation "buyers" does not exist
: ALTER TABLE "transactions" ADD CONSTRAINT "fk_rails_0b24a7fcc3"
FOREIGN KEY ("buyer_id")
REFERENCES "buyers" ("id")
C:/Users/Powerhouse/Desktop/CodingDojo/ruby/rails/demo/db/migrate/20161024032156_create_transactions.rb:3:in change'
C:inmigrate'
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: relation "buyers" does not exist
: ALTER TABLE "transactions" ADD CONSTRAINT "fk_rails_0b24a7fcc3"
FOREIGN KEY ("buyer_id")
REFERENCES "buyers" ("id")
C:/Users/Powerhouse/Desktop/CodingDojo/ruby/rails/demo/db/migrate/20161024032156_create_transactions.rb:3:in change'
C:inmigrate'
PG::UndefinedTable: ERROR: relation "buyers" does not exist
C:/Users/Powerhouse/Desktop/CodingDojo/ruby/rails/demo/db/migrate/20161024032156_create_transactions.rb:3:in change'
C:inmigrate'
Tasks: TOP => db:migrate
(See full trace by running task with --trace) }
Models
class User < ActiveRecord::Base
has_many :products
has_many :sales, :class_name => 'Transaction', :foreign_key => 'seller_id'
has_many :purchases, :class_name => 'Transaction', :foreign_key => 'buyer_id
end
Product Model
class Product < ActiveRecord::Base
belongs_to :user
end
Transaction model
class Transaction < ActiveRecord::Base
belongs_to :buyer, :class_name => 'User'
belongs_to :seller, :class_name => 'User'
belongs_to :product
end
Migration files
User
class CreateUsers < ActiveRecord::Migration
def change
create_table :users do |t|
t.string :name
t.timestamps null: false
end
end
end
Product
class CreateProducts < ActiveRecord::Migration
def change
create_table :products do |t|
t.references :user, index: true, foreign_key: true
t.string :name
t.timestamps null: false
end
end
end
Transaction
class CreateTransactions < ActiveRecord::Migration
def change
create_table :transactions do |t|
t.references :buyer, index: true, foreign_key: true
t.references :seller, index: true, foreign_key: true
t.references :product, index: true, foreign_key: true
t.timestamps null: false
end
end
end

Answer 1

This is due to PostgreSql not understanding how to make the custom association foreign keys.
Try changing the migration file to,
class CreateTransactions < ActiveRecord::Migration
def change
create_table :transactions do |t|
t.integer :buyer_id
t.integer :seller_id
t.integer :product_id
t.timestamps null: false
end
add_index(:transactions, :buyer_id)
add_index(:transactions, :seller_id)
add_index(:transactions, :product_id)
add_foreign_key :transactions, :users, column: :buyer_id
add_foreign_key :transactions, :users, column: :seller_id
end
end
It automatically links buyer_id to user, taking the power of
belongs_to :buyer, :class_name => 'User'
The above line makes the buyer_id as foreign key.
You can also try,
class CreateTransactions < ActiveRecord::Migration
def change
create_table :transactions do |t|
t.references :buyer, index: true
t.references :seller, index: true
t.references :product, index: true
t.timestamps null: false
end
add_foreign_key :transactions, :users, column: :buyer_id
add_foreign_key :transactions, :users, column: :seller_id
end
end

Answer 2

Your problem is that when ActiveRecord see this:
t.references :buyer, index: true, foreign_key: true
it tries to create a foreign key inside the database (because foreign_key: true) but the naming conventions tell AR that it should be linking to the buyers table (because you're referencing :buyer).
Just like you specify class_name: in your model:
belongs_to :buyer, :class_name => 'User'
to tell AR not use the conventional Buyer name you can override the convention in your migration by modifying your :foreign_key option:
t.references :buyer, index: true, foreign_key: { to_table: :users }
to explicitly name the table. Similarly for your other reference that doesn't use the conventional naming scheme:
t.references :seller, index: true, foreign_key: { to_table: :users }