Testing associations

robot_army_relation.rb

class Army < AR:B
  has_many :robots
  has_many :weapons
end

class Robot < AR:B
  belongs_to: :army
  has_one :weapon
end

# A weapon MAY belong to a robot if the army assigns it to him.
# * Is it better to validate this association by _id or model?
# * How to test? Can I inject a stub into army.robots relation to return an array?

class Weapon < AR:B
  belongs_to :army
  belongs_to :robot

  validates :army, presence: true
  validates :robot, inclusion: {in: proc { |w| w.army.robots }}, allow_nil: true
end