Add Set Default As SQL Function In Migration as a Rails TIL

rails/set-default-as-sql-function-in-migration.md

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+# Set Default As SQL Function In Migration
+
+With static default values, like `0`, `true`, or `'pending'`, we can set them
+directly as the value of `default`.
+
+```ruby
+class CreateActionsTable < ActiveRecord::Migration[7.2]
+  def change
+    create_table :actions do |t|
+      t.string :status, default: 'pending'
+    end
+  end
+end
+```
+
+However, if we want our default value to be a SQL function like `now()`, we
+have to use a lambda.
+
+Let's extend the above example to see what that looks like:
+
+```ruby
+class CreateActionsTable < ActiveRecord::Migration[7.2]
+  def change
+    create_table :actions do |t|
+      t.string :status, default: 'pending'
+
+      t.column :created_at, :timestamptz, default: -> { 'now()' }, null: false
+    end
+  end
+end
+```
+
+If we need to alter the default of an existing table's column, we can do
+something like this:
+
+```ruby
+class AddDefaultTimestampsToActions < ActiveRecord::Migration[7.2]
+  def up
+    change_column_default :actions, :created_at, -> { "now()" }
+    change_column_default :actions, :updated_at, -> { "now()" }
+  end
+
+  def down
+    change_column_default :actions, :created_at, nil
+    change_column_default :actions, :updated_at, nil
+  end
+end
+```
+
+I believe this functionality is available to Rails 5.0 and later.
+
+[source](https://github.com/rails/rails/issues/27077#issuecomment-261155826)