关于 rails ActiveRecord 属性 以及 foreign_key 不直接用数据库项目 时的一些讨论

belongs_to :major,:foreign_key => :major_id

但是数据库中并不直接有major_id这一项,而是从其他项表出[比如从一个xml项中解析出来]

最先想到的很简单

def major_id

end

def major_id=(m)

end

觉得这样实现就可以了,但是现实是残酷的

调用x.major时返回nil ,设置x.major = Major.find(1)时变量也不被设置

于是查看了ActiveRecord的源代码,:foreign_key实际读写的是x.attributes[name]项

也就是说自己定义的读取方法并不属于x.attributes

在API里查看到以下内容

Overwriting default accessors

All column values are automatically available through basic accessors on the Active Record object, but sometimes you want to specialize this behavior. This can be done by overwriting the default accessors (using the same name as the attribute) and calling read_attribute(attr_name) and write_attribute(attr_name, value) to actually change things. Example:

class Song < ActiveRecord::Base

# Uses an integer of seconds to hold the length of the song

def length=(minutes)

write_attribute(:length, minutes * 60)

end

def length

read_attribute(:length) / 60

end

end

You can alternatively use self[:attribute]=(value) and self[:attribute] instead of write_attribute(:attribute, value) and read_attribute(:attribute) as a shorter form.

尝试后发现:

如果设置x.major_id=...,可以使用x.major,一切正常

如果设置x.major,x.major_id也正常

但是

x = X.find(1)

x.major会出错
也就是说如果不调用读写方法,x.attributes[major_id] == nil,而foreign_key直接调用x.attributes[major_id]

只有调用钩子

def after_initialize

self[:major_id] =

end

这样就可以正常读取,写入

但对于写入方法,建议是在before_save钩子中完成,而不是使用覆盖属性读写的方法