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Resourcen:

• Episode 202: Active Record Queries in Rails 3
• Arel
• MetaWhere
• Full episode source code

Quellcode:

# rails console
Product.cheap.to_sql
(Category.joins(:products) & Product.cheap).to_sql
Category.with_cheap_products.to_sql
p = Product.discontinued.build
p.discontinued
t = Product.arel_table
t[:price].eq(2.99)
t[:name].matches("%catan").to_sql
Product.where(t[:price].eq(2.99).or(t[:name].matches("%catan")))

# models/product.rb
scope :discontinued, where(:discontinued => true)

def self.cheaper_than(price)
  where("products.price < ?", price)
end

scope :cheap, cheaper_than(5)

# models/category.rb
scope :with_cheap_products, joins(:products) & Product.cheap