Association belongs_to vs nested attributes in the form - ruby-on-rails

Question

I have a situation like below:
class User < ActiveRecord::Base
belongs_to :user_group
accepts_nested_attributes_for :user_group
end
class UserGroup < ActiveRecord::Base
has_many :users
end
Controller:
UsersController < ApplicationControler
def new
#user = User.new
#user.build_user_group
end
def create
#user = User.new(user_params)
if #user.save
# do something
else
# do something
end
end
private
def user_params
params.require(:user).permit(:email, :username, user_group_attributes: [:name])
end
end
Form:
= simple_form_for #user do |f|
= f.input :username
= f.simple_fields_for :user_group do |builder|
= builder.input :name, collection: UserGroup.all.map(&:name), prompt: "Choose one"
= f.input :email
= f.button :submit, 'Create', class: 'btn btn-success'
But it doesn't create an user with the association between the user and the user_group. UserGroup table is just a list of user groups, e.g. moderator, user and so on. So I need to select a group in the form and create a new user with association. What am I doing wrong? Do I need to find a group in create action and pass it as #user.user_group = the_chosen_group?
P.S.
Is it a proper name convention of UserGroup? Maybe should I call it as Group?
Regards.

Answer 1

Nested attributes should only be used when you want to allow editing of associated object via object itself. In short, every time you submit the form you created, rails will receive params like:
{ username: 'sth', user_group_attributes: { name: 'Group name' }
When you assign attributes like this, rails will create new attribute group, as it has no idea it is to search for such a group.
Since you only want to assign given user to usergrooup, you do not need nested_attributes at all. All you need is:
= simple_form_for #user do |f|
= f.input :username
= f.input :user_group_id, collection: UserGroup.all.pluck(:name, :id), prompt: 'Choose one'
= f.input :email
= f.button :submit, 'Create', class: 'btn btn-success'
And in the controller:
def user_params
params.require(:user).permit(:email, :username, :user_group_id)
end