Posted about 9 years ago. Visible to the public. Repeats.

Rails: When to use :inverse_of in has_many, has_one or belongs_to associations

When you have two models in a has_many, has_one or belongs_to association, the :inverse_of option in Rails tells ActiveRecord that they're two sides of the same association.

Example with a has_many / belongs_to association:

class Forum < ActiveRecord::Base has_many :posts, :inverse_of => :forum end class Post < ActiveRecord::Base belongs_to :forum, :inverse_of => :posts end

Knowing the other side of the same association Rails can optimize object loading so forum and forum.posts[0].forum will reference the same object in memory, instead of loading another copy of the same record.

Please read this great section about bi-directional associations Archive from the Rails API for details.

Rails >= 4

Starting from Rails 4 the inverse_of is tried to be determined automatically, but it has some caveats and doesn't work in all cases.

If you don't want to know the details, just set :inverse_of in all cases.

If you want to minimize your code, always set inverse of if you use any option.

You can find out if the inverse_of is set correctly:

Forum.reflect_on_association(:posts).has_inverse? # => false # missing and needs to be set manually Forum.reflect_on_association(:posts).has_inverse? # => :forum # looks good

Below there are some known options where a automatically determination of the inverse is not working:

Rails < 4

You should always use the :inverse_of option in Rails < 4.

Flaky tests are tests that sometimes fail for no obvious reason. They are the plague of many end-to-end (E2E) test suites that automate the browser through tools like Capybara and Selenium.

Join our free training event and learn to fix any flaky test suite, even in large legacy applications.

Owner of this card:

Henning Koch
Last edit:
4 days ago
by Henning Koch
About this deck:
We are makandra and do test-driven, agile Ruby on Rails software development.
License for source code
Posted by Henning Koch to makandra dev
This website uses short-lived cookies to improve usability.
Accept or learn more