module Sequel::Plugins::List::InstanceMethods

def at_position(p)

The model object at the given position in the list containing this instance. 
def at_position(p)
  list_dataset.first(position_field => p)
end

def before_create

position field already has a value.
Set the value of the position_field to the maximum value plus 1 unless the 
def before_create
  unless send(position_field)
    send("#{position_field}=", list_dataset.max(position_field).to_i+1)
  end
end

def last_position

Find the last position in the list containing this instance. 
def last_position
  list_dataset.max(position_field).to_i
end

def list_dataset

A dataset that represents the list containing this instance. 
def list_dataset
  model.scope_proc ? model.scope_proc.call(self) : model.dataset
end

def move_down(n = 1)

or 1 place if no argument is specified.
Move this instance down the given number of places in the list, 
def move_down(n = 1)
  move_to(position_value + n)
end

def move_to(target, lp = nil)

exception if target is less than 1 or greater than the last position in the list.
Move this instance to the given place in the list. Raises an 
def move_to(target, lp = nil)
  current = position_value
  if target != current
    checked_transaction do
      ds = list_dataset
      op, ds = if target < current
        raise(Sequel::Error, "Moving too far up (target = #{target})") if target < 1
        [:+, ds.filter(position_field=>target...current)]
      else
        lp ||= last_position
        raise(Sequel::Error, "Moving too far down (target = #{target}, last_position = #{lp})") if target > lp
        [:-, ds.filter(position_field=>(current + 1)..target)]
      end
      ds.update(position_field => Sequel::SQL::NumericExpression.new(op, position_field, 1))
      update(position_field => target)
    end
  end
  self
end

def move_to_bottom

Move this instance to the bottom (last position) of the list. 
def move_to_bottom
  lp = last_position 
  move_to(lp, lp)
end

def move_to_top

Move this instance to the top (first position, position 1) of the list. 
def move_to_top
  move_to(1)
end

def move_up(n = 1)

if no argument is specified.
Move this instance the given number of places up in the list, or 1 place 
def move_up(n = 1)
  move_to(position_value - n) 
end

def next(n = 1)

in the list, or 1 place below if no argument is given.
The model instance the given number of places below this model instance 
def next(n = 1)
  n == 0 ? self : at_position(position_value + n)
end

def position_field

The model's position field, an instance method for ease of use. 
def position_field
  model.position_field
end

def position_value

The value of the model's position field for this instance. 
def position_value
  send(position_field)
end

def prev(n = 1)

in the list, or 1 place below if no argument is given.
The model instance the given number of places below this model instance 
def prev(n = 1)
  self.next(n * -1)
end