Rails form_for multiple different class objects?

Typically something like this is done with associations of the two models. In which case, I will take a few assumptions here. I assume that you have two models user and pet. I assume that a pet belongs to a user and a user can have many pets. With this kind of association, a pet model would have a user_id attribute that would association the user to that pet.

User Model

attr_accessible :name, :pets_attributes
has_many :pets
accepts_nested_attributes_for :pets

Pet Model

attr_accesible :user_id, :name
belongs_to :user

Personally, I am a big fan of using simple_form as it has a cool association feature.

View

<% simple_form_for @user do |f| %>
  <%= f.input :name %>
  <%= f.simple_fields_for :pets do |p| %>
    <% p.input :name %>
  <% end %>
<% end %>

Controller

Within your controller, (going to assume that this is a new action). This will create a line for the user and build three spots for the pets.

def new
  @user = User.new
  3.times do
    @user.pets.build
  end
end

Routes

One thing that you may want to consider with the routes is to set them up similar to this. This would create your routes to be a bit prettier as a Pet show action would also be referenced to the user /users/:user_id/pets/:pet_id

resources :users do
  resources :pets
end