Question

It seems that I rarely, if ever, see or use a ruby constructor in a rails model.

My assumption is that because rails sets up so much stuff for you the need for initialization code is much lower.

Aren’t there any good use cases for a constructor in a model though?

Answer 1

There’s nothing wrong with a constructor, just that they’re hardly ever needed.
The main reason for a constructor would be for setting up default values. Setting default attributes, is easily done at the database level

add_column :users, :admin, :boolean, :default => false

The main other thing that might be needed would be the existence/creation of an association model, this can be done either in a constructor/initializer, but what’s more common is to use rails hooks to set a before_create :populate_children, :ensure_parent_exists (where populate_children, and ensure_parent_exists are private model methods) or something like that. This approach means that any initialization logic can be divided into logical methods (e.g. separate methods for each bit of initialization), and some can additionally be called at other times too after_save :ensure_parent_exists, and thus allows for more flexibility