An association defined with has_many :through will return the same record multiple times if multiple join models for the same record exist (a n:m relation). To prevent this, you need to add ->{ uniq } as second argument to has_many (below Rails 4 it is a simple option: has_many :xyz, :uniq => true).
Example
Say you have an Invoice with multiple Items. Each Item has a Product:
class Invoice < ActiveRecord::Base
has_many :items
has_many :products, :through => :items
end
class Item < ActiveRecord::Base
belongs_to :invoice
belongs_to :product
end
class Product < ActiveRecord::Base
has_many :items
end
Further say you have multiple items for the same product and invoice in the items table:
| id | invoice_id | product_id |
|---|---|---|
| 11 | 1001 | 55 |
| 12 | 1001 | 55 |
Now #products will return the product with ID #55 two times for this invoice:
invoice = Invoice.find(1001)
invoice.products # => [#<Product id: 55>, #<Product id: 55>]
You probably wanted Invoice#products to mean "all products involved with the invoice", so you need to use the :uniq option of has_many:
class Invoice < ActiveRecord::Base
has_many :items
has_many :products, :through => :items, :uniq => true
end
Now the product is only returned once:
invoice = Invoice.find(1001)
invoice.products # => [#<Product id: 55>]
How the :uniq option works
In the example above it changes the MySQL query for Invoice#products to something like
SELECT DISTINCT * FROM products ...
Issues with PostgreSQL
In Postgres, SELECT DISTINCT * FROM products ... will fail when any column of products is of type json:
ActiveRecord::StatementInvalid Exception: PG::UndefinedFunction: ERROR: could not identify an equality operator for type json
