Describe AMS architecture in the big picture [ci skip]

Author: bf4

docs/ARCHITECTURE.md

@@ -0,0 +1,118 @@
+# ARCHITECTURE
+
+An **`ActiveModel::Serializer`** wraps a [serializable resource](https://github.com/rails/rails/blob/4-2-stable/activemodel/lib/active_model/serialization.rb)
+and exposes an `attributes` method, among a few others.
+It allows you to specify which attributes and associations should represent the serialization of the resource.
+It requires an adapter to transform its attributes into a JSON document; it cannot be serialized itself.
+It may be useful to think of it as a
+[presenter](http://blog.steveklabnik.com/posts/2011-09-09-better-ruby-presenters).
+
+The **`ActiveModel::ArraySerializer`** represent a collection of resources as serializers
+and, if there is no serializer, primitives.
+
+The **`ActiveModel::Adapter`** describes the structure of the JSON document generated from a
+serializer. For example, the `Attributes` example represents each serializer as its
+unmodified attributes.  The `JsonApi` adatper represents the serializer as a [JSON
+API](jsonapi.org/) document.
+
+The **`ActiveModel::SerializableResource`** acts to coordinate the serializer(s) and adapter
+to an object that responds to `to_json`, and `as_json`.  It is used in the controller to
+encapsulate the serialization resource when rendered. Thus, it can be used on its own
+to serialize a resource outside of a controller, as well.
+
+## Primitive handling
+
+Definitions: A primitive is usually a String or Array. There is no serializer
+defined for them; they will be serialized when the resource is converted to JSON (`as_json` or
+`to_json`).  (The below also applies for any object with no serializer.)
+
+ActiveModelSerializers doesn't handle primitives passed to `render json:` at all.
+
+However, when a primitive value is an attribute or in a collection,
+it is is not modified.
+
+Internally, if no serializer can be found in the controller, the resource is not decorated by
+ActiveModelSerializers.
+
+If the collection serializer (ArraySerializer) cannot
+identify a serializer for a resource in its collection, it raises [`NoSerializerError`](https://github.com/rails-api/active_model_serializers/issues/1191#issuecomment-142327128)
+which is rescued in `AcitveModel::Serializer::Reflection#build_association` which sets
+the association value directly:
+
+```ruby
+reflection_options[:virtual_value] = association_value.try(:as_json) || association_value
+```
+
+(which is called by the adapter as `serializer.associations(*)`.)
+
+## How options are parsed
+
+High-level overview:
+
+- For a collection
+  - the collection serializer is the `:serializer` option and
+  - `:each_serializer` is used as the serializer for each resource in the collection.
+- For a single resource, the `:serializer` option is the resource serializer.
+- Options are partitioned in serializer options and adapter options.  Keys for adapter options are specified by
+    [`ADAPTER_OPTIONS`](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializable_resource.rb#L4).
+    The remaining options are serializer options.
+
+Details:
+
+1. **ActionController::Serialization**
+  1. `serializable_resource = ActiveModel::SerializableResource.new(resource, options)`
+    1. `options` are partitioned into `adapter_opts` and everything else (`serializer_opts`).
+      The adapter options keys for the are defined by `ADAPTER_OPTIONS`.
+1. **ActiveModel::SerializableResource**
+  1. `if serializable_resource.serializer?` (there is a serializer for the resource, and an adapter is used.)
+    - Where `serializer?` is `use_adapter? && !!(serializer)`
+      - Where `use_adapter?`: 'True when no explicit adapter given, or explicit value is truthy (non-nil);
+        False when explicit adapter is falsy (nil or false)'
+      - Where `serializer`:
+        1. from explicit `:serializer` option, else
+        2. implicitly from resource `ActiveModel::Serializer.serializer_for(resource)`
+  1. A side-effect of checking `serializer` is:
+     - The `:serializer` option is removed from the serializer_opts hash
+     - If the `:each_serializer` option is present, it is removed from the serializer_opts hash and set as the `:serializer` option
+  1. The serializer and adapter are created as
+    1. `serializer_instance = serializer.new(resource, serializer_opts)`
+    2. `adapter_instance = ActiveModel::Serializer::Adapter.create(serializer_instance, adapter_opts)`
+1. **ActiveModel::Serializer::ArraySerializer#new**
+  1. If the `serializer_instance` was a `ArraySerializer` and the `:serializer` serializer_opts
+    is present, then [that serializer is passed into each resource](https://github.com/rails-api/active_model_serializers/blob/a54d237e2828fe6bab1ea5dfe6360d4ecc8214cd/lib/active_model/serializer/array_serializer.rb#L14-L16).
+1. **ActiveModel::Serializer#attributes** is used by the adapter to get the attributes for
+  resource as defined by the serializer.
+
+## What does a 'serializable resource' look like?
+
+- An `ActiveRecord::Base` object.
+- Any Ruby object at passes or otherwise passes the
+  [Lint](http://www.rubydoc.info/github/rails-api/active_model_serializers/ActiveModel/Serializer/Lint/Tests)
+  [code](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model/serializer/lint.rb).
+
+ActiveModelSerializers provides a
+`[ActiveModelSerializers::Model](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model_serializers/model.rb)`,
+which is a simple serializable PORO (plain-old Ruby object).
+
+ActiveModelSerializers::Model may be used either as a template, or in production code.
+
+```ruby
+class MyModel < ActiveModelSerializers::Model
+  attr_accessor :id, :name, :level
+end
+```
+
+The default serializer for `MyModel` would be `MyModelSerializer` whether MyModel is an
+ActiveRecord::Base object or not.
+
+Outside of the controller the rules are **exactly** the same as for records. For example:
+
+```ruby
+render json: MyModel.new(level: 'awesome'), adapter: :json
+```
+
+would be serialized the same as
+
+```ruby
+ActiveModel::SerializableResource.new(MyModel.new(level: 'awesome'), adapter: :json).as_json
+```