Rewritten in longer form, assuming I've made no mistakes, the two versions are equivalent to...

  if params[:user_id].present?
    if !@owner
      @owner = User.find_by_login(params[:user_id])
    end
  end

vs

  if !@owner
    if params[:user_id].present?
      @owner = User.find_by_login(params[:user_id])
    end
  end

Checking the member variable truthiness is presumably faster than a dictionary lookup followed by a call to "present?". As @owner is probably frequently/always true after the first call (it will be unless find_by_login returns false/nil/? or another method resets @owner, given the same instance), this leads to fewer dictionary lookups and present? calls, resulting in a performance increase.

EDIT: Well that was an amusing spam of simultaneous answers.

I don't write Ruby, but it's close enough to Perl that i can tell you. Precedence is the reason.

The first line with more explicit precedence is this:

    if( params[:user_id].present?) {
        @owner ||= User.find_by_login(params[:user_id]) 
    }

The second line is this:

    @owner ||= (
        User.find_by_login(params[:user_id]) if params[:user_id].present?
    )

In other words, the first version always ran the present? method, while the second version only did so if @owner was false.

I haven't done too much Ruby myself, but the line looks sufficiently Perl like I believe the subtle change converted the first line (rolled out) from:

(excusing my Perl-Ruby mashup)

  if (params[:user_id].present){
    if (!@owner){
      @owner = User.find_by_login(params[:user_id]);
    }
  }

to:

  if (!@owner){
    if (params[:user_id].present){
      @owner = User.find_by_login(params[:user_id]);
    }
  }

Now the call params[:user_id].present is only made if @owner lacks a value.

Assignment has higher precedence than if, so the line originally would have evaluated as:

  (@owner ||= User.find_by_login(params[:user_id])) if params[:user_id].present?

Thus always testing the if, and then conditionally setting @owner. The new line checks the existence of @owner before evaluating the if by using parens to set precedence.

@owner ||= User.find_by_login(params[:user_id]) if params[:user_id].present? means:

  if params[:user_id].present?
    if not @owner
      @owner = User.find_by_login(params[:user_id])
    end
  end

@owner ||= (User.find_by_login(params[:user_id]) if params[:user_id].present?) means:

  if not @owner
    if params[:user_id].present?
      @owner = User.find_by_login(params[:user_id])
    end
  end

So everytime someone called owner() to get the owner of a repo, the regex was checked