Using build with a has_one association in rails.

Question

This is a really noob question but im having trouble finding the answer, is there a way in rails to have 0 or 1 association? For example, I create a user with no objects, than later on create an object for that user.

I tried using build with a 'has_one' association but that blew up... The only way I see this working is using 'has_many'. The user is suppose to only have at most one of these objects, is there any way to do this?

I have been trying this. I have:

class User < ActiveRecord::Base
  has_one :profile
end

class Profile < ActiveRecord::Base
  belongs_to :user
end

but when i do:

user.build_profile 

i get the error

ActiveRecord::StatementInvalid: Mysql::Error: Unknown column 'profiles.user_id' in 'where clause': SELECT * FROM `profiles` WHERE (`profiles`.user_id = 4)  LIMIT 1

Answer 1

It should be a has_one. If build isn't working, you can just use new:

ModelName.new( :owner => @owner )

is the same as

@owner.model_names.build

Answer 2

The build method signature is different for has_one and has_many associations.

class User < ActiveRecord::Base
  has_one :profile
  has_many :messages
end

The build syntax for has_many association:

user.messages.build

The build syntax for has_one association:

user.build_profile  # this will work

user.profile.build  # this will throw error

Read the has_one association documentation for more details.

Answer 3

Take a good look at the error message. It is telling you that you do not have required column user_id in the profile table. Setting the relationships in the model is only part of the answer.

You also need to create a migration that adds the user_id column to the profile table. Rails expects this to be there and if it is not you cannot access the profile.

For more information please take a look at this link: http://guides.rubyonrails.org/association_basics.html