What does inverse_of do? What SQL does it generate?

Question

From the documentation, it seems like the :inverse_of option is a method for avoiding SQL queries, not generating them. It's a hint to ActiveRecord to use already loaded data instead of fetching it again through a relationship.

Their example:

class Dungeon < ActiveRecord::Base
  has_many :traps, :inverse_of => :dungeon
  has_one :evil_wizard, :inverse_of => :dungeon
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon, :inverse_of => :traps
end

class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon, :inverse_of => :evil_wizard
end

In this case, calling dungeon.traps.first.dungeon should return the original dungeon object instead of loading a new one as would be the case by default.

I think :inverse_of is most useful when you are working with associations that have not yet been persisted. E.g.:

class Project < ActiveRecord::Base
  has_many :tasks, :inverse_of=>:project
end

class Task < ActiveRecord::Base
  belongs_to :project, :inverse_of=>:tasks
end

Now, in the console:

irb> p = Project.new
=> #<Project id: nil, name: nil, ...>
irb> t = p.tasks.build
=> #<Task id: nil, project_id: nil, ...>
irb> t.project
=> #<Project id: nil, name: nil, ...>

Without the :inverse_of arguments, t.project would return nil, because it triggers an sql query and the data isn't stored yet. With the :inverse_of arguments, the data is retrieved from memory.

After this pr (https://github.com/rails/rails/pull/9522) inverse_of is not required in most cases.

Active Record supports automatic identification for most associations with standard names. However, Active Record will not automatically identify bi-directional associations that contain a scope or any of the following options:

• :through
• :foreign_key

class Author < ApplicationRecord
  has_many :books, inverse_of: 'writer'
end

class Book < ApplicationRecord
  belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end

a = Author.first
b = a.books.first
a.first_name == b.writer.first_name # => true
a.first_name = 'David'
a.first_name == b.writer.first_name # => true

In the above example, a reference to the same object is stored in the variable a and in the attribute writer.

When we have 2 models with has_many and belongs_to relationship, it's always better to use inverse_of which inform ActiveRecod that they belongs to the same side of the association. So if a query from one side is triggered, it will cache and serve from cache if it get triggered from the opposite direction. Which improves in performance. From Rails 4.1, inverse_of will be set automatically, if we uses foreign_key or changes in class name we need to set explicitly.

Best article for details and example.

http://viget.com/extend/exploring-the-inverse-of-option-on-rails-model-associations