Rails Nested object Creationg - ruby-on-rails-4

Question

I have a project with the below scenario
Three Objects:
Client:The real world client
User: The who uses the portal
Department: The Client for whome the user works (XYZ.com,ABC.com)
The signup page have: FirstName,username,password,ClientName,Email
Client have only one field value: The client Name
FirstName,username,password,ClientName,Email are of user
Now what I want is to create a Department with ClientName as the Name of the Department and associate the user with the ID of the department.
User have a field department_id

Answer 1

Even with your reformulation, it is still not clear. So I'll go with my own interpretation. ALso, you didn't specify if you were using Mongoid or ActiveRecord. I'll go with a somewhat Mongoid style as it produces clean modelisation, but feel free to replace with ActiveRecord stuff.
Models
class User
has_one :department
has_one :client
accepts_nested_attributes_for :client
accepts_nested_attributes_for :department
field :first_name
field :password
field :username
field :email
class Client
belongs_to :user
field :name
class Department
belongs_to :user
field :name
your_controller.rb
def new
#user = User.new
#user.client = Client.new
#user.department = Department.new
end
def create
if #user = User.create(user_params)
redirect_to #user
else
render 'new'
end
end
private
def user_params
# Set department.name as client.name
if params[:user] and params[:user][:department] and params[:user][:client]
params[:user][:department][:name] = params[:user][:client][:name]
end
params.require[:user].permit(
:first_name, :password, :username, :email,
client_attributes: [:id, :name],
department_attributes: [:id, :name]
)
end
_form.html.erb
<%= form_for :user do |user| %>
<%= user.text_field(:name) %>
...
<%= user.fields_for :client do |client| %>
<%= client.text_field(:name) %>
<% end %>
<%= user.fields_for :department do |department| %>
<p class="text-info">A department will be created with the same name as the client</p>
<% end %>
<% end %>