How to set a default value for a datetime column to record creation time in a migration?

This is supported now in Rails 5.

Here is a sample migration:

class CreatePosts < ActiveRecord::Migration[5.0]
  def change
    create_table :posts do |t|
      t.datetime :modified_at, default: -> { 'CURRENT_TIMESTAMP' }
      t.timestamps
    end
  end 
end

See discussion at https://github.com/rails/rails/issues/27077 and answer there by prathamesh-sonpatki