has_and_belongs_to_many validations - ruby-on-rails-3

Question

What is the most straightforward way to check to make sure a creation of a new record includes the creation of a related record via has_and_belongs_to_many? For example, I have:
class Person < ActiveRecord::Base
has_and_belongs_to_many :groups
end
class Group < ActiveRecord::Base
has_and_belongs_to_many :people
end
I want a validation to fire on the creation of a new Person to make sure they belong to at least one group.
Also, how would I build this out in the controller? Right now I have:
def create
#person = current_user.people.new(params[:person])
end
I'd like params to include a group hash as well, to act as a sort of nested resource.
I've looked through the Rails documentation and I haven't been able to find anything on this particular case. If someone could explain this to me or point me in the right direction, I'd be very happy. Thanks!

Answer 1

If you want to give the user the option of creating one or more groups during the creation of a person, and then validate that those groups were created, please specify. Otherwise the remainder of this answer will be dedicated to creating a Person and validating that it is associated with at least one existing group.
If you're asking how to verify the existence of an Person-Group association on the groups_people join table, this could be done with weird sql queries and is inadvisable. Just trust that the well tested ActiveRecord works properly.
You can, however, validate the existence of one or more groups on a Person record before it is saved.
As long as you've migrated a join table called groups_people:
# db/migrate/xxxxxxxxxxxxxx_create_groups_people
class CreateGroupsPeople < ActiveRecord::Migration
def change
create_table :groups_people, :id => false do |t|
t.string :group_id, :null => false
t.string :person_id, :null => false
end
end
end
# $ rake db:migrate
, and your controller is correct:
# app/controllers/people_controller.rb
class PeopleController < ApplicationController
def new
#groups = Group.all
#person = Person.new
end
def create
#person= Person.new(params[:person])
if #person.save
# render/redirect_to and/or flash stuff
else
# render and/or flash stuff
end
end
end
, and you've all existing group options as checkboxes:
# app/views/people/new.html.erb
<%= form_for #person do |f| %>
<%= f.label :name %>
<%= f.text_field :name %>
# same for other person attributes
<% #groups.each do |g| %>
<%= check_box_tag 'person[group_ids][]', g.id, false, :id => g.group_name_attr %>
<%= label_tag g.group_name_attr %>
<% end %>
<%= f.submit 'Create!' %>
<% end %>
, then you can validate the presence of groups on your Person record:
class Person < ActiveRecord::Base
validates_presence_of :groups
has_and_belongs_to_many :groups
end
There is validates_associated helper, but wouldn't be necessary in this case, where you show Group.all as checkboxed options.
No accepts_nested_attributes_for is necessary for this. It would be if you were creating a Group for a Person while creating a Person. Again, please specify if this is the case.
Just a note: validating an incoming form that includes Group.all as options and gives the option of creating a group along with the person is possible but complicated. It would involve bypassing existing validations on the Group model, if any, which there probably is.