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module ActiveRecord
# This mixin provides the capabilities for sorting and reordering a number of objects in list.
# The class that has this mixin included needs to have a "position" column defined as an integer on
# the mapped database table. Further more, you need to implement the <tt>scope_condition</tt> that
# defines how to separate one list from another.
#
# Todo list example:
#
# class TodoList < ActiveRecord::Base
# has_many :todo_items, :order => "position"
# end
#
# class TodoItem < ActiveRecord::Base
# include ListMixin
# belongs_to :todo_list
#
# private
# def scope_condition
# "todo_list_id = #{todo_list_id}"
# end
# end
#
# todo_list.first.move_to_bottom
# todo_list.last.move_higher
#
module ListMixin
def self.append_features(base)
super
base.class_eval do
before_destroy :remove_from_list
after_create :add_to_list_bottom
end
end
# Moving around on the list
def move_lower
return unless lower_item
lower_item.decrement_position
increment_position
end
def move_higher
return unless higher_item
higher_item.increment_position
decrement_position
end
def move_to_bottom
decrement_positions_on_lower_items
assume_bottom_position
end
def move_to_top
increment_positions_on_higher_items
assume_top_position
end
# Entering or existing the list
def add_to_list_top
increment_positions_on_all_items
end
def add_to_list_bottom
assume_bottom_position
end
def remove_from_list
decrement_positions_on_lower_items
end
# Changing the position
def increment_position
self.position = position.to_i + 1
save
end
def decrement_position
self.position = position.to_i - 1
save
end
# Querying the position
def first?
self.position == 1
end
def last?
self.position == bottom_position_in_list
end
private
# Overwrite this method to define the scope of the list changes
def scope_condition; end
def higher_item
self.class.find_first(
"#{scope_condition} AND position = #{(position.to_i - 1).to_s}"
)
end
def lower_item
self.class.find_first(
"#{scope_condition} AND position = #{(position.to_i + 1).to_s}"
)
end
def bottom_position_in_list
item = bottom_item
item ? item.position : 0
end
def bottom_item
self.class.find_first(
"#{scope_condition} ",
"position DESC"
)
end
def assume_bottom_position
self.position = bottom_position_in_list.to_i + 1
save
end
def assume_top_position
self.position = 1
save
end
def decrement_positions_on_lower_items
self.class.update_all(
"position = (position - 1)", "#{scope_condition} AND position > #{position.to_i}"
)
end
def increment_positions_on_higher_items
self.class.update_all(
"position = (position + 1)", "#{scope_condition} AND position < #{position.to_i}"
)
end
def increment_positions_on_all_items
self.class.update_all(
"position = (position + 1)", "#{scope_condition}"
)
end
end
end
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