Handling Arrays of custom objects in Mongoid - ruby

Question

I'm having some issues with creating a Mongoid document that includes an array of custom objects.
I my particular case I intend to store an array of BaseDevice objects. The BaseDevice class is already mongified and serializes from/to a plain hash using Mongoid's custom fields support. This works pretty well on single object.
For storing an array of BaseDevice, I've created the following class:
class BaseDeviceArray < Array
class << self
def demongoize(object)
object ? object.map{ |obj| BaseDevice.demongoize obj } : new
end
def evolve(object)
case
when BaseDeviceArray then object.mongoize
else object
end
end
end
def mongoize
self.map(&:mongoize)
end
end
The mongoid document looks like this
class MongoPeriph
include Mongoid::Document
field :devices, type: BaseDeviceArray
end
Let's say some_devices is an array containing two BaseDevice instances.
What happens is the following: when I assign some_devices to the devices fields of the MongoPeriph instance that works correctly.
mp = MongoPeriph.create
mp.devices = some_devices
mp.devices # => [#<BaseDevice:0x007fa84bac0080>,#<BaseDevice:0x007fa84baaff78>]
When try to send push, pop, shift, unshift methods to the devices field within the mongoid document, nothing seems to happen. The changes are not appearing on the mp object. Also when referencing one of the objects by index (i.e. when calling mp.devices[0].some_method) the world does not change.
When popping objects from the array, on every pop a new object is given. This is expected as the deserializer is instantiating a new BaseDevice object for every pop, but the internal field is not updated i.e. the object stays there and one can pop endlessly.
Using the BaseDeviceArray separate from a mongoid document works as expected:
foo = BaseDeviceArray.new
foo << BaseDevice.new
results in an array with a BaseDevice object.
Btw. I found one other approach to this on the net. It is a more generalized way of implementing what I need, but it monkey-patches Mongoid. Something I try to avoid. Moreover that solution seems to have the same issue my approach has.

Answer 1

Issue in your code is that you have #mongoize (instance) method but you actually need ::mongoize (class) method. You never create an instance of BaseDeviceArray thus instance methods are useless.
Here's an example of how I did the ::mongoize method where I actually have in mongo a Hash with a single key with array value. Also I wanted to make the resulting array into a hash with ids as keys for easier lookup.
def demongoize(hash)
return validate_hash(hash)["TestRecord"].each_with_object({}) do |r, m|
rec = TestRecord.new(r)
m[rec.case_id] = rec
end
end
def mongoize(object)
case object
when Array then {"TestRecord" => object.map(&:mongoize)}
when Hash
if object["TestRecord"]
# this gets actually called when doing TestRun.new(hash)
mongoize(demongoize(object))
else
{"TestRecord" => object.values.map(&:mongoize)}
end
else raise("dunno how to convert #{object.class} into records JSON")
end
end
def evolve(object)
# can't see how we want to process this here yet
# docs.mongodb.com/ruby-driver/master/tutorials/6.0.0/mongoid-documents
object
end
I guess op task done long ago but thought somebody may find it useful.