Rails Form Action - ruby-on-rails

Question

I have a form like this in my html page
<%= form_for '/purchases', html: {class: "form form-horizontal validate-form", novalidate: "novalidate"} do |f| %>
But the form action is like this when i inspect the form
action = '/purchases/new'
I want the action to be just /purchases
and my controller method is like
#purchase = OrderItems.new

Answer 1

If you have your routes configured properly, you just need:
<%= form_for #purchase, html: {class: "form form-horizontal validate-form", novalidate: "novalidate"} do |f| %>
of course, provided you set #purchase in your new action:
#purchase = Purchase.new

Answer 2

I think you're getting confused between form_for and form_tag
--
form_for is for objects -
#app/controllers/purchases_controller.rb
Class PurchasesController < ApplicationController
def new
#purchase = Purchase.new
end
def create
#purchase = Purchase.new(purchase_params)
#purchase.save
end
private
def purchase_params
params.require(:purchase).permit(:purchase, :attributes)
end
end
The important thing to note with form_for is how it will build your form out of the ActiveRecord object you define. This is vitally important, and is at the root of your error:
<%= form_for #purchase do |f| %>
This will build all the different attributes of the form (including the action attribute), from the ActiveRecord object itself. This means if you populate your form_for with anything other than an object, you're going to get into trouble (as exhibited by your error)
--
form_tag is for data -
<%= form_tag your_path do %>
...
<% end %>
This might be better suited to your circumstances, as it allows you to create a "standalone" form - one which gives you the ability to send non-model-centric data to your application
We use form_tag implementations for the likes of search facilities etc
Solution
As Marek pointed out, you need to populate your form_for with an ActiveRecord object. To do this, you need first ensure you have initialized the object in your controller's new action, before passing the value to the form:
#app/controllers/purchases_controller.rb
Class PurchasesController < ActiveRecord::Base
def new
#purchase = Purchase.new
end
...
end
#app/views/purchases/new.html.erb
<%= form_for #purchase do |f| %>

Answer 3

I would just like to elaborate on the answers presented here. The way form_for works is it bases the form input names on the first argument passed to it. You can pass string or symbol as the first argument, and it should work, just like your example.
= form_for :purchase do |f|
= f.text_field :order
will result in a form that will submit to the current url and will contain 1 text field with the name purchase[order].
Marek's answer is correct however it doesn't answer your question. In cases where the object of the form, by convention, doesn't match the controller that will handle the request, you can pass a url option to form_for.
# controller
def new
#purchase = OrderItem.new
end
# view
= form_for #purchase, url: '/purchases' do |f|
= f.text_field :order
This will create a form with action set to /purchases but the name of the text field will be order_item[order].
FINAL NOTE
form_for is not exclusively used for active record objects as stated in the API docs