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Use timestamp values to represent boolean data such as deleted, published, or subscribed.
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Time for A Boolean

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Sally: Hey, we need to add a flag to Post

Jean: What for?

Sally: Well, we want to let users "delete" posts, but not actually lose the data.

Jean: Sounds reasonable. But what about later, when we have to know when a post was deleted?

Sally: That's a good point, but if we add a timestamp now we have to write all sorts of methods to keep a nice interface on Post...

Jean: Time for A Boolean!

Wait, what?

rails generate migration AddDeletedAtToPosts deleted_at:timestamp
class Post < ActiveRecord::Base
  time_for_a_boolean :deleted
  ...
end
class PostsController < ApplicationController
  def show
    @post = Post.find(params[:id])
    if @post.deleted?
      raise ActiveRecord::RecordNotFound
    end
  end

  def destroy
    post = Post.find(params[:id])
    post.deleted = true
    post.save
    redirect_to posts_url
  end
end

You keep on saying things and I don't get it.

Okay, let's take a look at what happens.

When we call time_for_a_boolean :deleted in the Post class definition, several methods are defined:

Method Description
Post#deleted true if Post#deleted_at is set to a time before Time.current, false otherwise
Post#deleted? Alias for Post#deleted
Post#deleted= Sets the timestamp to Time.current if the new value is true, and nil otherwise

These methods allow you to use a timestamp as you would a boolean value in your application.

Okay... why?

  • Audit for when a flag was set. Future you wants this.
  • COUNT(posts.deleted_at) gives you the count of deleted posts, which is useful when writing a report. Define and use Post.deleted.count when you have Ruby available.

Other Options

If you have a date or time column that does not follow the attribute_at convention, you can specify the attribute name:

class User < ActiveRecord::Base
  time_for_a_boolean :expires, :expires_on
end

This is especially useful when using date only columns.