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Martin Fowler: Embedded Document

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Flowing JSON data structures through a server is something I'm seeing more these days. JSON documents can be persisted directly, either by using an AggregateOrientedDatabase or a serialized LOB in a relational database. JSON documents can also be served directly to web browsers or used to transfer data to server-side page renderers. When JSON is being used in this way, I hear people saying that using an object-oriented language gets in the way because the JSON needs to be translated into objects only to be rendered out again - a waste of programming effort [1]. I agree with the point about waste, but I argue that it's not a problem with objects but a failure to understand encapsulation.

Let's imagine we're storing an order as a JSON document and serving it up with minor server-side processing, again as JSON. An example document might be like this.

{ "id": 1234,
  "customer": "martin",
  "items": [
    {"product": "talisker", "quantity": 500},
    {"product": "macallan", "quantity": 800},
    {"product": "ledaig",   "quantity": 1100}
  "deliveries": [
    { "id": 7722,
      "shipDate": "2013-04-19",
      "items": [
        {"product": "talisker", "quantity": 300},
        {"product": "ledaig",   "quantity": 500}
    { "id": 6533,
      "shipDate": "2013-04-18",
      "items": [
        {"product": "talisker", "quantity": 200},
        {"product": "ledaig",   "quantity": 300},
        {"product": "macallan", "quantity": 300}

We'll assume we haven't got much server-side processing to do, but we do have some. Let's also assume we're using an OO language. A naive approach might be to read in the JSON document, convert the data to the appropriate object graph (with orders, line-items, and deliveries), apply any processing, and then serialize the object graph to JSON for the client.

In many of these situtiations a better way to proceed is to keep the data in a JSONish form, but still wrap it with objects to coordinate manipulation. Most programming environments provide generic libraries that take a document and deserialize it to generic data structures. So a JSON document would deserialize to a structure of lists and dictionaries, an xml document to a tree of xml nodes. We can then take this generic data structure and put it into a field of an order object - here's an example with Ruby and JSON.

class Order...
  def initialize jsonDocument
    @data = JSON.parse(jsonDocument)

When we want to manipulate the data, we can define methods on the object as usual, and implement them by accessing this data structre.

class Order...
  def customer
  def quantity_for aProduct
    item = @data['items'].detect{|i| aProduct == i['product']}
    return item ? item['quantity'] : 0

This includes cases with more complex logic. [2]

class Order...
  def outstanding_delivery_for aProduct
    delivered_amount = @data['deliveries'].
      map{|d| d['items']}.
      select{|d| aProduct == d['product']}.
      inject(0){|res, d| res += d['quantity']}
    return quantity_for(aProduct) - delivered_amount

The embedded document can be enriched before sending to the client.

class Order...
  def enrich
    @data['earliestShipDate'] = 
      map{|d| Date.parse(d['shipDate'])}.

If needed, you can form similar objects on sub-trees of the embedded document.

class Order...
  def deliveries
    @data['deliveries'].map{|d| Delivery.new(d)}
class Delivery...
  def initialize hash
    @data = hash
  def ship_date

One thing be wary of here is that such object wrappers aren't quite the same as normal objects. The delivery objects returned in the above code fragment don't have the same equality semantics that you'd expect from objects arranged in the more usual structure.

Despite its compartive rareity, an embedded document fits well with object-orientation. The point of encapsulated data is the hiding of the data structure, so that users of the object don't know or care about the internal structure of the order.

Those familiar with functional programming will recognize the style of flowing a generic data structure through a series of functions - you can think of the object as providing a namespace for manipulating the generic data stuctures.

The sweet spot for an embedded document is when you're providing the document in the same form that you get it from the data store, but still want to do some manipulation of that data. If you don't have any need to access the contents of the JSON document, then there's no need to even deserialize it into a generic data structure. The order object needs only a constructor and a method to return its JSON representaiton. On the other hand as you do more work on the data - more server side logic, transforming into different representations - then it's worth considering whether it's easier to turn the data into an object graph.

1: Some might argue it's a waste of computing effort too - although I would be surprised if it were significant. I would certainly not accept a performance argument against converting to an object graph unless it was accompanied by measurements - just like any performance argument.

2: Note the chaining of CollectionLambdas in this method. One of my pet annoyances is hearing some functional fans say that this style of code isn't object-oriented. While it may seem foreign to those with a C++/Java background, this style is perfectly natural to smalltalkers.

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Published at DZone with permission of Martin Fowler, DZone MVB. See the original article here.

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