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Asked: 2026-06-01T13:24:23+00:00

I am wondering how to build a join table articles_categories, but one where there

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I am wondering how to build a join table articles_categories, but one where there can only be one category per article. I know the typical way to do categories is to allow multiple categories for each article with has_and_belongs_to_many, but I’d like only allow one category per article. I have already set up my articles_categories join table with the has_and_belongs_to_many associations. I followed an example from Apress Beginning Rails 3, that also recommended to set the articles_categories table :id => false. Will I also have to update the table to have an id? Thanks!

So far I have updated my articles model to has_one :category and the category model to has_many :articles, but I get an error while testing in irb: 

a = Article.first
a.category

The error is:

Category Load (0.7ms)  SELECT "categories".* FROM "categories" WHERE "categories"."article_id" = 1500 LIMIT 1
PG::Error: ERROR:  column categories.article_id does not exist
LINE 1: SELECT  "categories".* FROM "categories"  WHERE "categories"...

Can somebody tell me how to better set this up? Thanks!

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1 Answer

  1. Editorial Team

    If you want each article has only one category you should not use cross table. Simply use references in migration — http://guides.rubyonrails.org/migrations.html#highlighter_472212

    rails g migration ArticleHasCategory category:references

    Then you specify the article has category in model

    belongs_to :category

    When you call article.category. It will automatically look for column <relation name>_id which should point to record in <relation name>

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