Sql 活动记录：使用“连接查询”；其中；条款

这是我的模型：

class Book < ActiveRecord::Base
    attr_accessible :author, :title
    validates :author, presence: true
    validates :title, :uniqueness => true, presence: true

    has_many :rentals 

    def rent?
        rentals.where(return_date: nil).count > 0
    end
end

class Rental < ActiveRecord::Base
    belongs_to :student
    belongs_to :book

    validates :student, presence: true
    validates :book, presence: true
    validates :rental_date, presence: true

    attr_accessible :rental_date, :return_date, :student, :book

    def pending?
        return return_date == nil
    end

    def overdue?
        if(return_date)
            return_date - rental_date > 7.days
        else
            Time.now - rental_date > 7.days
        end
    end
end

但我还想添加一些基于参数的where查询

---

我怎样才能做到这一点呢？

您应该使用

连接
：

Book.joins('LEFT OUTER JOIN rentals ON books.id = rentals.book_id')
    .where('rentals.return_date IS NULL')

joins
方法将用于左外部联接，但您需要自己构造SQL片段。下面，结合
合并
和额外的租赁范围，演示了这一点

class Rental < ActiveRecord::Base
  scope :no_return, where('return_date IS NULL')
end

join_statement = 'LEFT OUTER JOIN rentals ON rentals.book_id = books.id'
Book.joins(join_statement).merge(Rental.no_return)
# => returns set of books with no rentals and 
# books with rentals that have no return date

但是用这种方式它会做一个内部连接，我需要一个外部连接。因此，我可以列出每一本没有归还日期的租赁书籍和从未租赁过的书籍。@DavidAnderson查看我的编辑，您可以使用带有
连接的外部连接。
class Rental < ActiveRecord::Base
  scope :no_return, where('return_date IS NULL')
end

join_statement = 'LEFT OUTER JOIN rentals ON rentals.book_id = books.id'
Book.joins(join_statement).merge(Rental.no_return)
# => returns set of books with no rentals and 
# books with rentals that have no return date

def pending?
  return_date.nil?
end