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| -rw-r--r-- | guides/source/association_basics.md | 16 |
1 files changed, 13 insertions, 3 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md index d83dda7228..520b324ea7 100644 --- a/guides/source/association_basics.md +++ b/guides/source/association_basics.md @@ -1872,11 +1872,21 @@ If you want to make sure that, upon insertion, all of the records in the persisted association are distinct (so that you can be sure that when you inspect the association that you will never find duplicate records), you should add a unique index on the table itself. For example, if you have a table named -`person_articles` and you want to make sure all the articles are unique, you could -add the following in a migration: +`readings` and you want to make sure the articles can only be added to a person once, +you could add the following in a migration: ```ruby -add_index :person_articles, :article, unique: true +add_index :readings, [:person_id, :article_id], unique: true +``` + +Once you have this unique index, attempting to add the article to a person twice +will raise an `ActiveRecord::RecordNotUnique` error: + +```ruby +person = Person.create(name: 'Honda') +article = Article.create(name: 'a1') +person.articles << article +person.articles << article # => ActiveRecord::RecordNotUnique ``` Note that checking for uniqueness using something like `include?` is subject |