Rails: finding database records - ruby-on-rails

Question

In order to improve my understanding of Rails, I'm converting a Sinatra app that uses data_mapper.
I'm trying to find the best replacements for data mappers 'first' method that searches database and returns first instance of the record sought.
Can anyone comment if this is done right, or if there's a better solution?
Situation #1
Sinatra
url = Url.first(:original => original)
Rails (both of these ok?)
url = Url.find_by_original(original) #this find_by_original
url = Url.where(:first_name => 'original')
situation #2
Sinatra
raise 'Someone has already taken this custom URL, sorry' unless Link.first(:identifier => custom).nil?
My Rails (with find)
raise 'Someone has already taken this custom URL, sorry' unless Link.find(:identifier => custom).nil? #this Link.find
Original context was a method that shortens urls
def self.shorten(original, custom=nil)
url = Url.first(:original => original)
return url.link if url
link = nil
if custom
raise 'Someone has already taken this custom URL, sorry' unless Link.first(:identifier => custom).nil?
raise 'This custom URL is not allowed because of profanity' if DIRTY_WORDS.include? custom
transaction do |txn|
link = Link.new(:identifier => custom)
link.url = Url.create(:original => original)
link.save
end
else
transaction do |txn|
link = create_link(original)
end
end
return link
end

Answer 1

You can just use a validator on the model. Just do a validates_uniqueness_of :first_name in the Url model (app/models/url.rb). There are also other validations available in the Rails guides
EDIT
If you really want to find if such a record exists manually. You can just do Url.find(:first, :conditions => { :first_name => 'original' }) and check for nil
raise 'Someone has already taken this custom URL, sorry' unless Link.find(:first, :conditions => { :identifier => custom }).nil?
Also, if you are new to rails might want to look at the query interface. Personally, I like to use the squeel gem so my queries are strictly ruby instead of mixing ruby and sql statements.