Question

I currently have 2 models setup:

class Topic < ActiveRecord::Base
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :topics
end

I am now trying to create a topic with a category associated in the rails console:

t = Topic.new :name => "Test", :category => Category.find(1)

Trouble is the model has category_id, and so I’d need to use:

c = Category.find(1)
t = Topic.new :name => "Test", :category_id => c.id

But, I’ve seen many times the ability to simply use :category instead of :category_id and pass in the category object instead of the objects id. Where am I going wrong?

When I do:

c = Category.find(1)
t = Topic.new :name => "Test", :category => c

I receive:

ActiveRecord::UnknownAttributeError: unknown attribute: category

Answer 1

You should be able to just do this:

c = Category.find(1)
t = Topic.new :name => "Test", :category => c

The association definition on the model is what lets you do this.

Interesting note, you can use :category_id and still just pass in the object, it will get the ID for you:

t = Topic.new :name => "Test", :category_id => c

Another way do do it which can be a bit nicer:

t = c.topics.build(:name => "Test") # Builds an object without saving

t = c.topics.create(:name => "Test") # Builds an object and saves it