Rails: How to limit number of items in has_many association (from Parent)

Question

I would like to limit the number of items in an association. I want to ensure the User doesn\'t have more than X Things. This question was asked before and the solution had th

Answer 1

In Rails 4, perhaps earlier versions you can simply validate on the value of the counter_cache.

class User
  has_many :things
  validates :things_count, numericality: { less_than: 5 }
end

class Thing
  belongs_to :user, counter_cache: true
  validates_associated :user
end

note that I've used :less_than because :less_than_or_equal_to would allow the things_count to be 6 since it is validated after the counter cache update.

If you want to set a limit on a per user basis, you can create a things_limit column to dynamically compare with the limit value you've set.

validates :things_count, numericality: { less_than: :things_limit }