User模型

 类用户的LT;的ActiveRecord :: Base的
  的has_many：medicalhistory
结束
 

Mdedicalhistory模型

 类Medicalhistory＆LT;的ActiveRecord :: Base的
  belongs_to的：用户#foreign键 - ＆GT;用户帐号
  用户：accepts_nested_attributes_for
结束
 

错误

 未定义的方法'姓'为＃＆LT;的ActiveRecord ::关联：0xb6ad89d0＆GT;


＃这个作品
@medicalhistory = Medicalhistory.find（current_user.id）
打印\ N+ @ medicalhistory.lastname

＃这个不！
@medicalhistory = Medicalhistory.where（user_ID的=？，current_user.id）
打印\ N+ @ medicalhistory.lastname #ERROR在这条线
 

解决方案

那么，你得到回的ActiveRecord ::关联的对象，而不是你的模型实例，从而的错误，因为有姓在的ActiveRecord ::关联。

做 @ medicalhistory.first.lastname 的作品，因为 @ medicalhistory.first 正在恢复模型的第一个实例被发现的其中，。

此外，您还可以打印出 @ medicalhistory.class 为工作和示数code，看看他们是如何的不同。

User model

class User < ActiveRecord::Base
  has_many :medicalhistory 
end

Mdedicalhistory model

class Medicalhistory < ActiveRecord::Base
  belongs_to :user #foreign key -> user_id
  accepts_nested_attributes_for :user
end

Error

undefined method `lastname' for #<ActiveRecord::Relation:0xb6ad89d0>


#this works
@medicalhistory = Medicalhistory.find(current_user.id) 
print   "\n" + @medicalhistory.lastname

#this doesn't!
@medicalhistory = Medicalhistory.where("user_id = ?", current_user.id)
print   "\n" + @medicalhistory.lastname #error on this line

Well, you are getting back an object of ActiveRecord::Relation, not your model instance, thus the error since there is no method called lastname in ActiveRecord::Relation.

Doing @medicalhistory.first.lastname works because @medicalhistory.first is returning the first instance of the model that was found by the where.

Also, you can print out @medicalhistory.class for both the working and "erroring" code and see how they are different.