How do I access the previous record in a rails block

So basically I need to return the value of the previous Point.

e.g. I'm on the third point. I need the point.kilometric_position of the second point.

Part of my controller action is pasted here. Thanks in advance!

def calculate
  @points = Point.all

  # loop through all the points
  @points.each do |point|
    # calculate the current kilometric position
    if point.kilometric_开发者_开发问答position.nil?
      # kilometric_position = previous_kilometric_position + distance
    end
  end
end

@points.each_with_index do |point, i|
  previous_point = @points[i-1] unless i==0

  next unless previous_point
  distance = point.distance_to(previous_point)
  # do something with distance
end

If you want to use a more ruby friendly method, use inject like this:

@points.inject do |previous,current|
  previous  # first time through, this is the first value
  current   # first time through, this is the second value

  # do a bunch of stuff

  previous = current
end

Would this be alright, seems inefficient but it's all I can think of right now:

@previous_point = @points[@points.index(point)-1]

I might do

def calculate
  # Why were you using @points rather than points?
  points = Point.all

  current_kilometric_position = 0
  # loop through all the points
  points.each do |point|
    point.kilometric_position = current_kilometric_position
    current_kilometric_position += distance
  end
end

but your question is too vague - it doesn't specify what distance is, why you only want points with kilometric_position equalling nil to have their position calculated, and what the kilometric_position of the first point should be.