have your to_param begin with the object's id

If you overwrite the to_param method in your model class such that it does not begin with its id, you can be in for a nasty surprise:

Example

class User
  def to_param
    self.login
  end
  ...
end

Let’s say you have a user called “bob”, than you might think this works:

>> bob = User.find(3)
=> #<User id: 3, login: "bob", ...>
>> User.find(bob.to_param)
ActiveRecord::RecordNotFound: Couldn't find User with ID=bob

But it’s not the reason being that Rails find method looks for a beginning number (d+) and uses that to look up the record (and ignores everything that comes after the last digit). So the solution is to have your to_param return something that begins with the object’s id, like so:

Example

class User
  def to_param
    "#{self.id}-#{self.login}"
  end
  ...
end

>> bob = User.find(3)
=> #<User id: 3, login: "bob", ...>

>> User.find(bob.to_param)

> # id: 3, login: “bob”, …>

>> bob.to_param

> “3-bob”