Promote important architecture description that answers a lot of questions we get

README.md

@@ -156,7 +156,141 @@ serializer = SomeSerializer.new(resource, serializer_options)
 serializer.attributes
 serializer.associations
 ```
-See [ARCHITECTURE.md](docs/ARCHITECTURE.md) for more information.
+
+## Architecture
+
+This section focuses on architecture the 0.10.x version of ActiveModelSerializers. If you are interested in the architecture of the 0.8 or 0.9 versions,
+please refer to the [0.8 README](https://github.com/rails-api/active_model_serializers/blob/0-8-stable/README.md) or
+[0.9 README](https://github.com/rails-api/active_model_serializers/blob/0-9-stable/README.md).
+
+The original design is also available [here](https://github.com/rails-api/active_model_serializers/blob/d72b66d4c5355b0ff0a75a04895fcc4ea5b0c65e/README.textile).
+
+### ActiveModel::Serializer
+
+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 be represented in the serializatation 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).
+
+#### ActiveModel::CollectionSerializer
+
+The **`ActiveModel::CollectionSerializer`** represents a collection of resources as serializers
+and, if there is no serializer, primitives.
+
+### ActiveModelSerializers::Adapter::Base
+
+The **`ActiveModelSerializeres::Adapter::Base`** 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` adapter represents the serializer as a [JSON
+API](http://jsonapi.org/) document.
+
+### ActiveModelSerializers::SerializableResource
+
+The **`ActiveModelSerializers::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. However, it can also 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.
+
+Internally, if no serializer can be found in the controller, the resource is not decorated by
+ActiveModelSerializers.
+
+- However, when a primitive value is an attribute or in a collection, it is not modified.
+
+When serializing a collection and the collection serializer (CollectionSerializer) cannot
+identify a serializer for a resource in its collection, it throws [`:no_serializer`](https://github.com/rails-api/active_model_serializers/issues/1191#issuecomment-142327128).
+For example, when caught by `Reflection#build_association`, and the association value is set 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**
+  - `:serializer` specifies the collection serializer and
+  - `:each_serializer` specifies 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_OPTION_KEYS`](https://github.com/rails-api/active_model_serializers/blob/master/lib/active_model_serializers/serializable_resource.rb#L5).
+    The remaining options are serializer options.
+
+Details:
+
+1. **ActionController::Serialization**
+  1. `serializable_resource = ActiveModelSerializers::SerializableResource.new(resource, options)`
+    1. `options` are partitioned into `adapter_opts` and everything else (`serializer_opts`).
+      The `adapter_opts` keys are defined in [`ActiveModelSerializers::SerializableResource::ADAPTER_OPTION_KEYS`](lib/active_model_serializers/serializable_resource.rb#L5).
+1. **ActiveModelSerializers::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::CollectionSerializer#new**
+  1. If the `serializer_instance` was a `CollectionSerializer` 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.
+
+(In Rails, the `options` are also passed to the `as_json(options)` or `to_json(options)`
+methods on the resource serialization by the Rails JSON renderer.  They are, therefore, important
+to know about, but not part of ActiveModelSerializers.)
+
+### What does a 'serializable resource' look like?
+
+- An `ActiveRecord::Base` object.
+- Any Ruby object that 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 reference implementation, or in production code.
+
+```ruby
+class MyModel < ActiveModelSerializers::Model
+  attributes :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
+ActiveModelSerializers::SerializableResource.new(MyModel.new(level: 'awesome'), adapter: :json).as_json
+```
 
 ## Semantic Versioning
 

docs/README.md

@@ -18,7 +18,6 @@ This is the documentation of ActiveModelSerializers, it's focused on the **0.10.
 - JSON API
   - [Schema](jsonapi/schema.md)
   - [Errors](jsonapi/errors.md)
-- [ARCHITECTURE](ARCHITECTURE.md)
 
 ## How to
 

docs/general/rendering.md

@@ -48,26 +48,11 @@ render json: @posts, serializer: CollectionSerializer, each_serializer: PostPrev
 
 ## Serializing non-ActiveRecord objects
 
-All serializable resources must pass the
-[ActiveModel::Serializer::Lint::Tests](../../lib/active_model/serializer/lint.rb#L17).
-
-See the ActiveModelSerializers::Model for a base class that implements the full
-API for a plain-old Ruby object (PORO).
+See [README](../../README.md#what-does-a-serializable-resource-look-like)
 
 ## SerializableResource options
 
-The `options` hash passed to `render` or `ActiveModelSerializers::SerializableResource.new(resource, options)`
-are partitioned into `serializer_opts` and `adapter_opts`. `adapter_opts` are passed to new Adapters;
-`serializer_opts` are passed to new Serializers.
-
-The `adapter_opts` are specified in [ActiveModelSerializers::SerializableResource::ADAPTER_OPTIONS](../../lib/active_model_serializers/serializable_resource.rb#L5).
-The `serializer_opts` are the remaining options.
-
-(In Rails, the `options` are also passed to the `as_json(options)` or `to_json(options)`
-methods on the resource serialization by the Rails JSON renderer.  They are, therefore, important
-to know about, but not part of ActiveModelSerializers.)
-
-See [ARCHITECTURE](../ARCHITECTURE.md) for more information.
+See [README](../../README.md#activemodelserializersserializableresource)
 
 ### adapter_opts