Mongoid many-to-many with versioning - ruby-on-rails

Question

Assume, that a user wants to create a set consisting of items created by other users. Mongoid Document for item has versioning, and the user who creates the set might not enjoy changes the item authors do with the items of the set. Therefore I would like the set document to refer to specific versions of items, allowing the set author to update item references if wanted. I am planning adding an array of item version numbers to the set document, and some methods for getting set items of certain version and for updating the item versions. Do you find this approach reasonable? How would you solve this problem?
class Item
include Mongoid::Document
include Mongoid::Paranoia
include Mongoid::Versioning
field :title, type: String
has_and_belongs_to_many :item_sets
end
class ItemSet
include Mongoid::Document
field :name, type: String
field :item_versions, type: Array
has_and_belongs_to_many :items
end

Answer 1

I solved problems like this by creating a model in the "middle" like "ItemReference"
MongoDB is a document store and not a relational database, so it is legitimate to store duplicate information when necessary. MongoDB has the ability to store embedded documents, so we're gonna use this great feature.
The ItemReference holds all crucial information about the Item which is needed for creating a view. This reduces the queries on the view side but in increases the queries on the insert/update side.
The thing is that you need a "composite primary key" that consists of the item_id and the version number.
Let's talk code:
The Item Model:
class Item
include Mongoid::Document
include Mongoid::Paranoia
include Mongoid::Versioning
field :title, :type => String
# create a reference
def to_reference
# create new reference, containing all crucial attributes for displaying
ItemReference.new(
:item_id => self._parent.nil? ? self.id : self._parent.id,
:version => self.version,
:title => self.title
)
end
# get a certain version of this item
def get_version(version_number)
return self if version_number == self.version
self.versions.where(:version => version_number).first
end
end
The ItemSet Model
class ItemSet
include Mongoid::Document
field :name, :type => String
embeds_many :item_references
end
The ItemReference Model
class ItemReference
include Mongoid::Document
embedded_in :item_sets
field :title, :type => String
# this points to the newest version
belongs_to :item
# get the original version
def original
self.item.get_version(self.version)
end
# update this reference to a certain version
def update_to!(new_version)
new_version = self.item.get_version(new_version)
if new_version.present?
# copy attribute, except id
self.attributes = new_version.to_reference.attributes.reject{|(k,v)| k == "_id"}
self.save
else
# version not found
false
end
end
# update to the newest version
def update_to_head!
self.update_to!(self.item.version)
end
end
This combination allows you to create sets holding Items with different versions, and you can update certain ItemReferences in the set to a specific version.
Here's an example:
first = Item.create(:title => 'Item 1')
first.title = 'Item 1.1'
first.save
myset = ItemSet.create(:title => 'My Set')
myset.item_references << first.to_reference
myset.save
first.title = 'Item 1.2'
first.save
p myset.item_references.first.title # prints Item 1.1
p myset.item_references.first.update_to_head!
p myset.item_references.first.title # prints Item 1.2
p myset.item_references.first.update_to!(1)
p myset.item_references.first.title # prints Item 1