Question

我一直在玩铁轨，而且我很难理解当我把东西插入模型时，外键如何自动填入模型表中。例如，在Cart与Ingredients的1：N表关系中（假设没有制作推车），如果我编码：

# create cart and couple of ingredients
cart1 = Cart.create()
jelly = Ingredient.create()
pb = Ingredient.create()

# push jelly, and pb, into cart1
cart1.ingredients << jelly
cart1.ingredients << pb

# pb and jelly cart_id is now populated
pb.cart_id
>>1
jelly.cart_id
>>1

除了将配料推入购物车之外，rails如何填充外国ID，cart_id，列，而不需要做任何事情？

Answer 1

你可以想象这样的过程：

# creates cart with id=1
cart = Cart.create

# creates ingredient with card_id=nil
jelly = Ingredient.create

# It's one-to-many relation, which knows that
# the name of the foreign key is cart_id.
# How does it know?
# Rails uses conventional logic here:
# if you don't name things explicitly then Rails
# will guess them for you.
# Name of the model is Cart, so the foreign key is cart_id.
cart.ingredients

# One-to-many relation receives some object.
# Relation asks object: do you have method cart_id=?
# Object replies: yes, I have
# Relation then takes primary key (id in this case)
# from cart (=1) and places it into object's cart_id field.
cart.ingredients << jelly

jelly.cart_id # => 1

Answer 2

作为per：

按照惯例，Rails假定用于保存外来的列此模型上的键是带有后缀_id的关联的名称添加。 :foreign_key选项允许您设置外部名称直接关键

这虽然专门针对belongs_to关联进行了描述，但解释了Rails如何处理foreign_keys。

您的系统中的每个对象都可以由ActiveRecord ORM 关联。如果您有has_many，belongs_to或has_and_belongs_to_many关系，ActiveRecord将根据自己的逻辑填充外键（如上所述），或者如果您在模型中明确设置它。 / p>

所以当你有：

@cart.ingredients #-> ActiveRecord method populates foreign key of child objects with cart_id

当我将创建的模型推入其中时，rails如何自动分配foreign_key？

2 个答案: