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:

Copyclass 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 Show archive.org snapshot 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:

CopyForum.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:

• :conditions, :through and :foreign_key: See the Ruby On Rails Guide Show archive.org snapshot and the INVALID_AUTOMATIC_INVERSE_OPTIONS constant for each Rails version respectively (e.g. for Rails v5.2.3 Show archive.org snapshot ).
• :as: Association to polymorphic model does not determine inverse_of automatically.

Rails < 4

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