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

Rails >= 4

Starting from Rails 4 the inverse_of is tried to be determined automatically.

  1. If you don't use options setting the inverse_of is not needed.
  2. If the inverse is wrong or missing because of some options, you need to set the inverse_of manually.

Our recommendation is to always set inverse of if you use any option. Still 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

When you have two models in a has_many, has_one or belongs_to association, you should always use the :inverse_of option in Rails < 4 to tell 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

By setting this option, 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. This will save you countless headaches.

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

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Author of this card:

Henning Koch
Last edit:
about 1 month ago
by Florian Leinsinger
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Posted by Henning Koch to makandra dev