诗歌rails之嵌套使用rails find :include选项

在Rails的API里，find方法有如下参数，conditions order group having limit offset joins include select from readonly lock

   其中，对于include的解释是说，include选项后的参数，应该是一个已经关联的表。该表信息会在检索后，和检索表信息一起加载。给的例子如下：

Person.find(:all, :include => [ :account, :friends ])

Person.find(:all, :include => [ :account, :friends ])

   include选项的好处就不举例说了，其实，就和你用游标做全检索，还是join出一个view一个道理。

    现在说下面一个情况，有学生表和班级表如下：

class Class < ActiveRecord::Base
    has_many :students
end

class Student < ActiveRecord::Base
    belongs_to :class
end

class Class < ActiveRecord::Base
    has_many :students
end

class Student < ActiveRecord::Base
    belongs_to :class
end

   
    很显然，当我们要输出所有学生的班级信息时，我们会用include如下：

Student.find(:all,:include => [:class], :conditions =>["user_name = ?", user_name])

Student.find(:all,:include => [:class], :conditions =>["user_name = ?", user_name])

    然而，当我们的要求增加，有一个新的表学校信息也希望检索出来时，那么，表结构如下：

class University < ActiveRecord::Base
         has_many :class
end
class Class < ActiveRecord::Base
         has_many :students
         belongs_to :university
end

class Student < ActiveRecord::Base
    belongs_to :class
end

class University < ActiveRecord::Base
         has_many :class
end
class Class < ActiveRecord::Base
         has_many :students
         belongs_to :university
end

class Student < ActiveRecord::Base
    belongs_to :class
end

     那当我们想要输出所有学生的学校相关信息时，那么，也许我们会想到使用检索语句

Student.find(:all,:include => [:class,:university], :conditions =>["student_name = ?", student_name])

Student.find(:all,:include => [:class,:university], :conditions =>["student_name = ?", student_name])

     实际上，运行结果是，我们会得到一个':university' is not a valid association of the Student model的错误提示。

      这个错误提示，是可以理解的。正如，文章开始的Rails API里所说的include后面的参数应该是一个已经和检索表有关联的表。实际上University和Student表没有直接关联。这时的惯性思维是使用find_by_sql,诚然，的确可以解决。其实，应该嵌套include选项。

      那么，解决这个问题需要用到through参数。更新表结构如下：

class University < ActiveRecord::Base
         has_many :class
end

class Class < ActiveRecord::Base
         has_many :students
         belongs_to :university
end

class Family < ActiveRecod::Base
         has_many :students
end

class Student < ActiveRecord::Base
    belongs_to :class
         belongs_to :university, :through => :class
         belongs_to :family
end

class University < ActiveRecord::Base
         has_many :class
end

class Class < ActiveRecord::Base
         has_many :students
         belongs_to :university
end

class Family < ActiveRecod::Base
         has_many :students 
end

class Student < ActiveRecord::Base
    belongs_to :class
         belongs_to :university, :through => :class
         belongs_to :family
end

      执行检索语句：

Student.find(:all,:include => [{:university=>:class},:family], :conditions =>["student_name = ?", student_name])

Student.find(:all,:include => [{:university=>:class},:family], :conditions =>["student_name = ?", student_name])

请注意其中嵌套部分的使用。

（本文参考：http://snippets.dzone.com/posts/show/2089）