max max -4 years ago 117
Ruby Question

How do I check if an argument is an instance of model or the class in Rails?

I have a helper method which generates "crud" links for a resource.
Right now it only handles edit and destroy. But I want to refactor it so that it would create a

link if passed a model class instead of an instance.

Desired output:

> crud_links_for_resource(Product)
> { :new => '<a href="/products/new">Create a new product.</a>'}

What is the best way to check if a variable is the class of a model or an instance? I thought of using duck typing (
resource.respond_to? :new_record?
) but is there a better way?

module PermissionsHelper
# Generates a hash of links to edit or destroy a resource
# @param [ActiveModel] resource
# @param [Actions] a list of crud actions to create links to
# @param [Hash] kwargs optional hash to pass to link to
# @option kwargs [String] :controller - controller name to use.
# Otherwise a guess is performed based on the resource class name.
# @option kwargs [Hash] url_extras - passed to url_for. Can be used for nested resources.
# @return [Hash] a list of links to actions which the user is allowed to perform
def crud_links_for_resource(resource, actions = [:destroy, :edit], **kwargs)
privledges = actions.keep_if { |action| can? action, resource }
privledges.each_with_object({}) do |action, hash|
i18n_key = resource.model_name.i18n_key
txt = t("#{ i18n_key }.#{action}")
controller = kwargs[:controller] || i18n_key.to_s.pluralize
url_extras = kwargs[:url_extras] || {}
options = kwargs.except(:controller, :url_extras)
case action
when :destroy
options.merge!(method: :delete, confirm: t("#{ i18n_key }.confirm_#{action}"))
hash[action] = link_to(txt, { action: action, controller: controller, id: resource }.merge(url_extras) ,options)

Answer Source
if myobject.is_a?(Foo)
  #it's an instance of foo
  #it's something else


if myobject.is_a?(Class)
  #it's a class
  #it's not
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