Rails is rife with magic. Form helpers, in particular the ability to build forms directly linked to Ruby objects, are one piece of Rails magic that I had a hard time wrapping my head around.
This is a quick guide to setting up nested forms in rails.
First - what is a nested form? It is, as it’s name suggests, a form within a form. More specifically, the nested form is creating or manipulating a model that different from the model the parent form is for.
Let’s pretend we have two models - owners, and dogs. An owner can have many dogs, and a dog belongs to one owner.
class Owner < ActiveRecord::Base
has_many :dogs
end
and
class Dog < ActiveRecord::Base
belongs_to :owner
end
The form to create a new dog would look something like this:
<h1>Create a new Dog:</h1>
<%= form_for(@dog) do |f|%>
<div>
<%= f.label :name%>
<%= f.text_field :name%>
</div><br>
<div>
<%= f.label :breed%>
<%= f.text_field :breed%>
</div><br>
<div>
<%= f.label :age%>
<%= f.text_field :age%>
</div><br>
<div>
<label for="dog_owner_id">Select an owner:</label>
<%= f.collection_select :owner_id, owner.all, :id, :name, prompt: true%>
</div><br>
<%= f.submit %>
<% end %>
The above form gives a the user the ability to select and owner from owners already saved to the database - but what if they need to create a new owner? It would be a terrible user experience to have the user realize this, then go to a different page to create an owner, then back to this page to create the dog. Nested forms to the rescue!
Step 1 - Model
Add the accepts_nested_attributes_for method to the model that our main form is for, in our example, Dogs. This is a class method that defines an attribute writer for the specified association (owners). You can read up on it here.
Our Owner class now looks like this:
class Dog < ActiveRecord::Base
belongs_to :owner
accepts_nested_attributes_for :owner
end
Step 2 - View
Next, we need to update our view by adding the nested form. To do this, we’ll use fields_for, which, like form_for yields a FormBuilder object.
<div>
<h3>Or create a new owner:</h3>
<%= f.fields_for :owner, Owner.new do |owner_attributes|%>
<%= owner_attributes.label :name, "Owner Name:" %>
<%= owner_attributes.text_field :name %>
<% end %>
</div>
Our updated form looks like this:
<h1>Create a new Dog:</h1>
<%= form_for(@dog) do |f|%>
<div>
<%= f.label :name%>
<%= f.text_field :name%>
</div><br>
<div>
<%= f.label :breed%>
<%= f.text_field :breed%>
</div><br>
<div>
<%= f.label :age%>
<%= f.text_field :age%>
</div><br>
<div>
<label for="dog_owner_id">Select an owner:</label>
<%= f.collection_select :owner_id, owner.all, :id, :name, prompt: true%>
</div><br>
<div>
<h3>Or create a new owner:</h3>
<%= f.fields_for :owner, Owner.new do |owner_attributes|%>
<%= owner_attributes.label :name, "Owner Name:" %>
<%= owner_attributes.text_field :name %>
<% end %>
</div>
<%= f.submit %>
<% end %>
Step 3a - Controller
When a user submits the original form, the controller action is to create a new instance of a dog. With the nested form, we also need to create a new instance of an owner, and associate that owner with the dog. This is accomplished with the build method.
class DogsController < ApplicationController
def new
@dog = Dog.new
@dog.build_owner
end
end
Step 3b - Controller
We’ll also need to update our strong parameters in the controller. Using accepts_nested_attributes_for in the Dog model added a new attribute to the class.
class DogsController < ApplicationController
def dog_params
params.require(:dog).permit(:name, :breed, :age owner_attributes: [:name])
end
end