Rails: How can I set default values in ActiveRecord?

We put the default values in the database through migrations (by specifying the :default option on each column definition) and let Active Record use these values to set the default for each attribute.

IMHO, this approach is aligned with the principles of AR : convention over configuration, DRY, the table definition drives the model, not the other way around.

Note that the defaults are still in the application (Ruby) code, though not in the model but in the migration(s).

The Phusion guys have some nice plugin for this.

This is what constructors are for! Override the model's initialize method.

Use the after_initialize method.

I ran into problems with after_initialize giving ActiveModel::MissingAttributeError errors when doing complex finds:

eg:

@bottles = Bottle.includes(:supplier, :substance).where(search).order("suppliers.name ASC").paginate(:page => page_no)

"search" in the .where is hash of conditions

So I ended up doing it by overriding initialize in this way:

def initialize
  super
  default_values
end

private
 def default_values
     self.date_received ||= Date.current
 end

The super call is necessary to make sure the object initializing correctly from ActiveRecord::Base before doing my customize code, ie: default_values

class Item < ActiveRecord::Base
  def status
    self[:status] or ACTIVE
  end

  before_save{ self.status ||= ACTIVE }
end

Rails 6.1+

You can now use the attribute method on your model without setting a type.

attribute :status, default: ACTIVE

or

class Account < ApplicationRecord
  attribute :locale, default: 'en'
end

Note that feeding a default to attribute cannot reference the instance of the class (a lambda will execute in the context of the class, not the instance). So, if you need to set the default to a value dynamically based on the instance or associations, you're still going to have to use an alternative, such as an after_initialize callback. As stated previously, it's recommended to limit this to new records only to avoid n+1 queries if you reference associations.

after_initialize :do_something_that_references_instance_or_associations, if: :new_record?

If the column happens to be a 'status' type column, and your model lends itself to the use of state machines, consider using the aasm gem, after which you can simply do

  aasm column: "status" do
    state :available, initial: true
    state :used
    # transitions
  end

It still doesn't initialize the value for unsaved records, but it's a bit cleaner than rolling your own with init or whatever, and you reap the other benefits of aasm such as scopes for all your statuses.

Here's a solution I've used that I was a little surprised hasn't been added yet.

There are two parts to it. First part is setting the default in the actual migration, and the second part is adding a validation in the model ensuring that the presence is true.

add_column :teams, :new_team_signature, :string, default: 'Welcome to the Team'

So you'll see here that the default is already set. Now in the validation you want to ensure that there is always a value for the string, so just do

 validates :new_team_signature, presence: true

What this will do is set the default value for you. (for me I have "Welcome to the Team"), and then it will go one step further an ensure that there always is a value present for that object.

Hope that helps!

# db/schema.rb
create_table :store_listings, force: true do |t|
  t.string :my_string, default: "original default"
end

StoreListing.new.my_string # => "original default"

# app/models/store_listing.rb
class StoreListing < ActiveRecord::Base
  attribute :my_string, :string, default: "new default"
end

StoreListing.new.my_string # => "new default"

class Product < ActiveRecord::Base
  attribute :my_default_proc, :datetime, default: -> { Time.now }
end

Product.new.my_default_proc # => 2015-05-30 11:04:48 -0600
sleep 1
Product.new.my_default_proc # => 2015-05-30 11:04:49 -0600

I had a similar challenge when working on a Rails 6 application.

Here's how I solved it:

I have a Users table and a Roles table. The Users table belongs to the Roles table. I also have an Admin and Student Models that inherit from the Users table.

It then required that I set a default value for the role whenever a user is created, say admin role that has an id = 1 or student role that has an id = 2.

class User::Admin < User
  before_save :default_values

  def default_values
    # set role_id to '1' except if role_id is not empty
    return self.role_id = '1' unless role_id.nil?
  end
end

This means that before an admin user is created/saved in the database the role_id is set to a default of 1 if it is not empty.

return self.role_id = '1' unless role_id.nil? 

is the same as:

return self.role_id = '1' unless self.role_id.nil?

and the same as:

self.role_id = '1' if role_id.nil?

but the first one is cleaner and more precise.

That's all.

I hope this helps

Been using this for a while.

# post.rb
class Post < ApplicationRecord
  attribute :country, :string, default: 'ID'
end

use default_scope in rails 3

api doc

ActiveRecord obscures the difference between defaulting defined in the database (schema) and defaulting done in the application (model). During initialization, it parses the database schema and notes any default values specified there. Later, when creating objects, it assigns those schema-specified default values without touching the database.

discussion