Validate association count in Rails

Say you have a Post model that should have a maximum of 3 Tags asscotiated to it. There are multiple ways to achieve that:

Use the built in `length` validation

In models/Post.rb:

has_many :tags
validates :tags, length: { maximum: 3 }

The default error message for this would similar to "tags too long". This sounds weird, so the translations should be adapted: en.activerecord.errors.models.posts.attributes.tags.too_long

Custom validation

  has_many :tags
  validate :tags_count

  def tags_count
    errors.add(:tags_count, "not more than three tags allowed") if tags.size > 3
  end

The adventage here is that no translations are needed.
Since almost all applications I work on are multilanguage and have to be translated, I prefer the first method. It is also more readable.

Chris Jarling