>
> I've added some accessors because I don't know if they are needed  
> by ActiveRecord, but nothing changes.
>
> class Player < ActiveRecord::Base
>    
> attr_reader :team_id, :name, :age, :goal, :defence, :stamina, :goalkee 
> per,
>               :playmaking, :pass, :shot, :speed, :head, :creativity, : 
> free_kicks,
>               :captain, :mood, :aggresiveness
>    
> attr_writer :team_id, :name, :age, :goal, :defence, :stamina, :goalkee 
> per,
>               :playmaking, :pass, :shot, :speed, :head, :creativity, : 
> free_kicks,
>               :captain, :mood, :aggresiveness
>   belongs_to :team
>
>   def initialize
>     # De momento ponemos cualquier cosa, un numero aleatorio como  
> cadena
>     @name = (rand * 1000000).truncate.to_s
>     # Edad entre 20 y 34 añïs
>     @age = rand(34) + 1 + 20
>   end
>
>   # Crea un jugador con los valores por defecto
>   def Player.create_player(team_id)
>     player = Player.new
>     player.team_id = team_id
>     player.save
>   end
>
>   # Relaciona el jugador con un equipo
>   def relate_to_team(team_id)
>     @team_id = team_id
>   end
> end
>

just try

class Player < ActiveRecord::Base
   belongs_to :team
end

1 - you don't need to specify properties, ActiveRecord gets them from  
the table definition, so no attr_reader etc
2 - you don't need to add a method to save the player, as it inherits  
all the methods from ActiveRecord

If you can save a standard ActiveRecord-based object, then you can  
change it and add instance/class methods later

--
"Man is truly free only among equally free men" - Michael Bakunin