Checking if a record exists in Sinatra/DataMapper

I currently generate a user's profile page using their serial ID, like so:

get '/users/:id' do
  @user = User.get(params[:id])
end

This works great, until a number is entered that doesn't exist in the database.

I'm aware I can change User.get to User.get! to return an ObjectNotFoundError error if the record isn't found, but I'm not sure how I can use this to my aid.

I used to use .exists? when using RoR.

Any suggestions?

Thanks in advance!

Edit: I'm going to leave the question unanswered, as I haven't actually found a solution to what I asked in the title; however, I did manage to solve my own problem by checking to see if the :id entered is higher than the amount of users that exist in the database, like so:

if params[:id].to_i > User.count
  "This user does not exist."
else
  @users_id = User.get(params[:id])
  erb(:'users/id')
end

Solution

You already have the correct code:

@user = User.get(params[:id])

If no user exists with the given id, then #get will return nil. Then you can just do a conditional:

@user = User.get params[:id]
if @user
  # user exists
else
  # no user exists
end

This is a very common pattern in Ruby which takes advantage of the "truthiness" of anything other than false or nil. i.e. you can say if 0 or if [] and the condition will evaluate to true

You can recreate a .exists? method:

class User
  def self.exists?(id_or_conditions)
    if id_or_conditions.is_a? Integer
      !! User.get id_or_conditions
    else
      !! User.first id_or_conditions
    end
  end
end

#get is similar to #find in rails, except it doesn't raise an error if the record is not found. #first is similar to #find_by in rails.