til/rails/params-is-a-hash-with-indifferent-access.md

Params Is A Hash With Indifferent Access

When you have an instance of ActionController::Parameters—which you often do with params in a Rails controller—under the hood you have a HashWithIndifferentAccess.

Here is what the initializer looks like when you call ActionController::Parameters.new(some_hash).

def initialize(parameters = {}, logging_context = {})
  @parameters = parameters.with_indifferent_access
  @logging_context = logging_context
  @permitted = self.class.permit_all_parameters
end

This means you can reference the keys in your parameters as either a string key or a symbol key.

> params = ActionController::Parameters.new({ username: 'tacocat' })
=> #<ActionController::Parameters {"username"=>"tacocat"} permitted: false>
> params['username']
=> "tacocat"
> params[:username]
=> "tacocat"

Note that ActiveSupport::HashWithIndifferentAccess is not an ancestor of the ActionController::Parameters class (like it once used to be), but is just how the incoming hash is transformed when initialized.