Default values for model fields in a Ruby on Rails form

Question

I have a Model which has fields username, data, tags, date, votes. I have form using form_for that creates a new item and puts it into the database. However, as you can guess I want the votes field to equal 0 and the date field to equal the current date when it is placed into the database. How and where would I set/apply these values to the item?

I can get it to work with hidden fields in the form but this comes with obvious issues (someone could set the votes field to a massive number).

Answer 1

Just use a default value; zero, for votes in the db, use the automatic timestamps(created_at) instead of date, and have fields in the form only for the parameters you will set. Don't forget to protect the sensitive attributes.

class CreateModels < ActiveRecord::Migration
  def up
    create_table :models do |t|
      t.string :username
      t.text :data
      t.string :tags
      t.integer :votes, :default => 0

      t.timestamps # this will give you two automatic fields: created_at and updated_at
    end
  end
  …
end

class Model < ActiveRecord::Base
  attr_protected :votes #so that it cannot be set by mass assignment
  …
end

Answer 2

I can confirm that DB constraints approach is one of the best. But it is not always possible to be used. Assume there is a single table inheritance and different default value per child model is required. Than I recommend to put this into a model. Let me give an example:

class ChildModel < Model
  after_initialize :set_defaults
  private
  def set_defaults
    self.allowed_votes_per_person = 10 if self.new_record?
  end 
end

By using :after_initialize callback there is no need to create and remember to call :new_default like methods and so on. It sets required default values, but stays unobtrusive from the interface point of view when one calls ChildModel.new