Frozen hash?

Rails
1

Hi, having this models:

class Formation < ActiveRecord::Base
has_many :positions, :dependent => :destroy
end

class TeamFormation < Formation
has_many :players, :through => :positions
end

class Position < ActiveRecord::Base
belongs_to :formation
belongs_to :player
end

I’m trying to modify position’s player attribute from a formation:

team.players.each do |player|
  self.positions.each do |position|
    if !self.players.include?(player) and !position.player
      position.player = player <------------ HERE I'M TRYING TO

MODIFY PLAYER
break
end
end
end

The first time I create a formation an asign those values all works
fine, but once it’s created I
can’t modify position’s player attr because “can’t modify frozen hash”
error is raised.

How could I modify those values?, what am I doing wrong?