Howto save / update using multiple tables?

Rails
#1

Controller

def create
@person = Person.new(params[:person])
@address = Address.new(params[:address])

…
end

If a person has an address, how can I save some data into 2 different
tables?

Can I do the following?

@person.address = @address
if @person.save
render …
else
render_text “Error”
end

Or do I have to use:

if @person.save
if @address.save
render …
else
render_text “Error”
else
render_text “Error”
end

#2

I found this example that it is supposed to be working

def add_player
case @request.method;
when :get
@team = Team.find(params[:id])
@player = Player.new
when :post
@team = Team.find(params[:id])
@player = Player.new(params[:player])
@team.players << @player
if @team.save
flash[:notice] = ‘Player was successfully added.’
redirect_to :action => ‘show’, :id => params[:id]
else
render :action => ‘add_player’
end
end
end

Unfortunately the << is for arrays. Then I have tried something like:
@team.player = @player
if @team.save
…
else
…
end

Now the error is “Cannot add or update a child row: a foreign key
constraint fails”

What does that mean? Any ideas?