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.