How to save to two models at once using Rails and has_many through association?

Question

In my rails 3.2 site I have Users and Communities. When a User creates a new Community I want the User to also automatically become a Member of that Community. Memberships form another model. The relationships are as follows:

User:

has_many :memberships
has_many :communities, :through => :memberships

Community:

has_many :memberships
has_many :users, :through => :memberships

Membership:

belongs_to :user
belongs_to :community

In a nutshell then, how do I create a Membership at the same time as I create a Community using the user_id and membership_id obviously?

Update

Controller action:

def create
  @community = Community.new(params[:community])

  respond_to do |format|
  if @community.save
    format.html { redirect_to @community, notice: 'Community was successfully created.' }
    format.json { render json: @community, status: :created, location: @community }
    else
    format.html { render action: "new" }
    format.json { render json: @community.errors, status: :unprocessable_entity }
  end
end

end

Answer 1

The working answer was this:

def create
  @community = current_user.communities.create(params[:community])

  respond_to do |format|
    if @community ...

The problem was using build and then @community.save. This did not save the association. Using .create automatically calls new and save and creates the association.

Answer 2

This should work if a user creates a community like this:

user.communities.create

From the Rails guide:

The collection of join models can be managed via the API. For example, if you assign

physician.patients = patients

new join models are created for newly associated objects, and if some are gone their rows are deleted.