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

You can add a function in a model like this:

  before_create :set_foo_to_now
  def set_foo_to_now
    self.foo = Time.now
  end

So that the model will set the current time in the model.

You can also place some sql code in the migration for setting the default value at the database level, something like:

execute 'alter table foo alter column starts_at set default now()'

Setting something like this:

create_table :foo do |t|
  t.datetime :starts_at, :null => false, :default => Time.now
end

causes executing the Time.now function during migrating so then the table in database is created like this:

create table foo ( starts_at timestamp not null default '2009-01-01 00:00:00');

but I think that it is not what you want.

Active Record automatically timestamps create and update operations if the table has fields named created_at/created_on or updated_at/updated_on. Source - api.rubyonrails.org

You don't need to do anything else except to have that column.