PostgreSQL，Rails 3 - 如何在关联中添加新的外键？

Question

我正在研究这个项目，现在我需要添加一个外键。我有这两种模式：类

class Owner < ActiveRecord::Base 
    has_many :shops 
end 

class Shop < ActiveRecord::Base 
    belongs_to :owner 
    has_many :cities 
end 

class City < ActiveRecord::Base 
    belongs_to :shop, :foreign_key => 'shop_sid' 
end 

我的目标是打印所有的城市，其中是商店。我试图做的是这样的：

<% @owner.shops.each do |shop| %> 
    <% shop.cities.each do |city| %> 
    <%=city.position%><br /> 
    <%end%> 
<%end> 

But I am getting the error 
PG::Error: ERROR: column cities.shop_id does not exist 
LINE 1: ...T "cities".* FROM "cities" WHERE "cities... 
                  ^
: SELECT "cities".* FROM "cities" WHERE "cities"."shop_id" = 8 

我也试图与外键添加到cities表：

alter table cities add foreign key (shop_sid) references shops (sid); 

，但得到

ERROR: there is no unique constraint matching given keys for referenced table "shops" 

我怎么能解决这个问题关于外键的问题？或者，更新协会也会更好？

PG::Error: ERROR: operator does not exist: character varying = integer 
LINE 1: ...ons" WHERE "cities"."shop_sid" = 1034 
                   ^
HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. 
: SELECT "cities".* FROM "cities" WHERE "cities"."shop_sid" = 1034 

Answer 1

你需要指定关系中每一侧的外键： -

编辑错误信息

class Shop < ActiveRecord::Base 
    belongs_to :owner 
    has_many :cities, :foreign_key => 'shop_sid' 
end 

class City < ActiveRecord::Base 
    belongs_to :shop, :foreign_key => 'shop_sid' 
end