Limit number of objects in has_many association

In my case, it was sufficient to use validates_length_of:

class Album
  has_many :photos
  validates_length_of :photos, maximum: 10
end

class Photo
  belongs_to :album
  validates_associated :album
end

Use a validation hook:

class Album
  has_many :photos
  validate_on_create :photos_count_within_bounds

  private

  def photos_count_within_bounds
    return if photos.blank?
    errors.add("Too many photos") if photos.size > 10
  end
end

class Photo
  belongs_to :album
  validates_associated :album
end

How about adding a custom validation method to the Photo model?

  LIMIT = 50

  validate_on_create do |record|
    record.validate_quota
  end

  def validate_quota
    return unless self.album
    if self.album.photos(:reload).count >= LIMIT
      errors.add(:base, :exceeded_quota)
    end
  end