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Automatically initialize associations in your Rails models
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Automatically initialize associations in your Rails 3 models.


AutoBuild gives your models the option to automatically initialize (build) their associations. This is useful to skip the build_association or calls in your models, controllers or views when working with nested fields.


Add auto_build to your Gemfile:

gem "auto_build"

And run bundle install.


has_one associations

Just call the auto_build method in your models:

 class User
   has_one :address
   has_one :phone

   auto_build :address

With this in place, address will always be initialized, so you don't have to manually call build_address anywhere. If the User already has an address assigned this won't overwrite it.

You can also do:

auto_build :address, :phone,

To initialize several fields. One after_initialize callback will be created per association.

belongs_to associations

Works the same as the has_one association:

 class User
   belongs_to :directory

   auto_build :directory

has_many associations

You can automatically initialize a has_many association in the same way you initialized a has_one association:

class User
  has_many :friends
  auto_build :friends

# == 1

The behavior will be the same, it will create a new Friend if none exists yet. If there's at least one Friend in the collection then nothing will be created.

If you want to always append a new empty object at the end of the collection, you can pass the :append => true option:

class User
  has_many :friends
  auto_build :friends, :append => true

# count = some_user.friends.count
# user = User.find(
# user.friends.count == count + 1

This will always create a new Friend instance regardless of the value of friends. It won't overwrite any existing values.

Finally, if you want to always bulk-add a number of records, you can pass the :count option with the number of records you want to add:

class User
  has_many :friends
  auto_build :friends, :count => 5

# count = some_user.friends.count
# user = User.find(
# user.friends.count == count + 5

This will always add 5 extra objects at the end of the collection. It won't overwrite any existing values.


  • Watch out for fields with reject_if. Since the AutoBuild callback will be added as an after_initialize hook this might overwrite the validations done in reject_if.
  • The option :append => true is equivalent to :count => 1.
  • None of the operations will overwrite existing objects.

Autobuilding associations means that if there's a value in the column, the object will be loaded every time you load the parent. This is problematic if you're trying to optimize your code. To get a better picture of this:

class User
  has_one :address
  auto_build :address

If you do the resulting query will be SELECT id FROM users; plus SELECT * from addresses WHERE user_id = 42";. This happens because we're calling calling user#address every time we initialize a User object.

How it works

auto_build works by adding an after_initialize callback per association to your model.


To report bugs or suggest new features go to the Issues tracker.

This was written by Federico. You can follow me on Twitter @febuiles. Let me know if you find this useful.

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