Rails的问题归属于

Question

Hey guys just trying to figure out why this does not work. Basically this relationship does

belongs_to :product_category, :foreign_key => :category_id

And this one does not

belongs_to :category, :class_name => :product_category, :foreign_key => :category_id

The error message is "NameError: uninitialized constant product::product_category"

Why is that? Thanks!

Answer 1

The latter example does not work because there is no class called product_category . You are providing the wrong class name. Class names in Ruby should be written in CamelCase. When Rails looks for a product_category class it's not going to find it.

Your first example works because Rails infers the name of the class from the name of the relationship.

belongs_to :product_category, :foreign_key => :category_id

It converts product_category to ProductCategory . You can do the same thing yourself. Open up a terminal and type the following.

'product_category'.camelize.constantize

You should pass in a string instead:

belongs_to :category, :class_name => 'ProductCategory', :foreign_key => :category_id

But in this case it would be redundant since Rails can already infer the class name. The class_name argument should be used when the class name can not be inferred from the relationship name.