ActiveRecord_Associations_CollectionProxy的未定义方法'id_tag'

Question

In my CrawlProduct class I want to get the id_tag from the table ProductInfo:

class CrawlProduct < ActiveRecord::Base
    belongs_to :crawl_link

    css_id = self.crawl_link.domain.product_infos.id_tag
end

The table CrawlProducts has a foreign key crawl_link and the table CrawlLinks has a foreign key domain which comes from the table Domain of course. And ProductInfo belongs to domain.

class CrawlProduct < ActiveRecord::Base
        belongs_to :crawl_link 
end   

class CrawlLink < ActiveRecord::Base
        belongs_to :domain
        has_one :crawl_product
end

class Domain < ActiveRecord::Base
        has_many :crawl_links
        has_many :product_infos
end

class ProductInfo < ActiveRecord::Base
        belongs_to :domain
end

But at the end, I always receive the following error:

undefined method `id_tag' for #<ProductInfo::ActiveRecord_Associations_CollectionProxy:0x0055d72e423640>

How can I access the id_tag ? There is no column missing as far as I know.

Edit : after the answers I get a different error.

NameError: undefined local variable or method `id_tag' for #<CrawlProduct:0x0055d72fcd9fe0>

I don't know why I'm getting this problem. Here is how I create the id_tag:

class Domain < ActiveRecord::Base
   domain = Domain.create(domain: 'https://www.example.com')
   product_info = ProductInfo.create(domain: domain, id_tag: 'some content')
end

Answer 1

self.crawl_link.domain.product_infos is a collection of instances, so you can't call instance methods on it, to call it on each item you should either iterate collection with each or collect ids with map . To collect just ids without creating instances you can use pluck :

self.crawl_link.domain.product_infos.pluck(:id_tag)

Answer 2

self.crawl_link.domain.product_infos will return a group of objects so, you cannot retrieve id_tag(a field) from a collection.

Instead you can use,.

class CrawlProduct < ActiveRecord::Base
    belongs_to :crawl_link

    def some_action
      css_ids = self.crawl_link.domain.product_infos.pluck(:id_tag)

      (or)

      css_ids = self.crawl_link.domain.product_infos.map(&:id_tag)
    end
end

Answer 3

I think you're accessing product_infos incorrectly. You need to specify which one you want, and then access the id_tag from it.

Rails is correctly telling you that

undefined method `id_tag' for #<ProductInfo::ActiveRecord_Associations_CollectionProxy:0x0055d72e423640>

because the relationship is has_many.

If you wanted to just access the first one the database pulls out (in no specific order, unless you specify), you could just do

css_id = self.crawl_link.domain.product_infos.first.id_tag