Integrate with Active Model Attributes

The `schema { ... }` interface pre-dates the Active Model Attributes API
(defined as early as [v5.2.0][]), but clearly draws inspiration from
Active Record's Database Schema and Attribute casting (which was
extracted into `ActiveModel::Attributes`).

However, the type information captured in `schema { ... }` blocks or
assigned as `Hash` arguments to `schema=` is purely inert metadata.

Proposal
---

This commit aims to integrate with [ActiveModel::Model][] and
[ActiveModel::Attributes][]. Through the introduction of both modules,
subclasses of `ActiveResource::Schema` can benefit from type casting
attributes and constructing instances with default values.

This commit makes minimally incremental changes, prioritizing backwards
compatibility. The reliance on `#respond_to_missing?` and
`#method_missing` is left largely unchanged. Similarly, the `Schema`
interface continues to provide metadata about its attributes through the
`Schema#attr` method (instead of reading from
`ActiveModel::Attributes#attribute_names` or
`ActiveModel::Attributes.attribute_types`).

API Changes
---

To cast values to their specified types, declare the Schema with the
`:cast_values` set to true.

```ruby
class Person < ActiveResource::Base
  schema cast_values: true do
    integer 'age'
  end
end

p = Person.new
p.age = "18"
p.age # => 18
```

To configure inheriting resources to cast values, set the `cast_values`
class attribute:

```ruby
class ApplicationResource < ActiveResource::Base
  self.cast_values = true
end

class Person < ApplicationResource
  schema do
    integer 'age'
  end
end

p = Person.new
p.age = "18"
p.age # => 18
```

To set all resources application-wide to cast values, set
`config.active_resource.cast_values`:

```ruby
  # config/application.rb
  config.active_resource.cast_values = true
```

[v5.2.0]: https://api.rubyonrails.org/v5.2.0/classes/ActiveModel/Attributes/ClassMethods.html
[ActiveModel::Model]: https://api.rubyonrails.org/classes/ActiveModel/Model.html
[ActiveModel::Attributes]: https://api.rubyonrails.org/classes/ActiveModel/Attributes/ClassMethods.html

Author: seanpdoyle

lib/active_resource/base.rb

@@ -381,6 +381,12 @@ def self.logger=(logger)
     class_attribute :connection_class
     self.connection_class = Connection
 
+    class_attribute :cast_values, instance_accessor: false, instance_predicate: false # :nodoc:
+    self.cast_values = false
+
+    class_attribute :schema_definition, instance_accessor: false, instance_predicate: false # :nodoc:
+    self.schema_definition = Schema
+
     class << self
       include ThreadsafeAttributes
       threadsafe_attribute :_headers, :_connection, :_user, :_password, :_bearer_token, :_site, :_proxy
@@ -441,16 +447,48 @@ def new_lazy_collections=(value)
       #
       # Attribute-types must be one of: <tt>string, text, integer, float, decimal, datetime, timestamp, time, date, binary, boolean</tt>
       #
-      # Note: at present the attribute-type doesn't do anything, but stay
-      # tuned...
-      # Shortly it will also *cast* the value of the returned attribute.
-      # ie:
-      # j.age                 # => 34   # cast to an integer
-      # j.weight              # => '65' # still a string!
+      # Note: By default, the attribute-type is ignored and will not cast its
+      # value.
+      #
+      # To cast values to their specified types, declare the Schema with the
+      # +:cast_values+ set to true.
+      #
+      #   class Person < ActiveResource::Base
+      #     schema cast_values: true do
+      #       integer 'age'
+      #     end
+      #   end
+      #
+      #   p = Person.new
+      #   p.age = "18"
+      #   p.age # => 18
+      #
+      # To configure inheriting resources to cast values, set the +cast_values+
+      # class attribute:
       #
-      def schema(&block)
+      #   class ApplicationResource < ActiveResource::Base
+      #     self.cast_values = true
+      #   end
+      #
+      #   class Person < ApplicationResource
+      #     schema do
+      #       integer 'age'
+      #     end
+      #   end
+      #
+      #   p = Person.new
+      #   p.age = "18"
+      #   p.age # => 18
+      #
+      # To set all resources application-wide to cast values, set
+      # +config.active_resource.cast_values+:
+      #
+      #   # config/application.rb
+      #   config.active_resource.cast_values = true
+      def schema(cast_values: self.cast_values, &block)
         if block_given?
-          schema_definition = Schema.new
+          self.schema_definition = Class.new(schema_definition)
+          schema_definition.cast_values = cast_values
           schema_definition.instance_eval(&block)
 
           # skip out if we didn't define anything
@@ -490,6 +528,7 @@ def schema(&block)
       def schema=(the_schema)
         unless the_schema.present?
           # purposefully nulling out the schema
+          self.schema_definition = Schema
           @schema = nil
           @known_attributes = []
           return
@@ -1336,6 +1375,7 @@ def known_attributes
     def initialize(attributes = {}, persisted = false)
       @attributes     = {}.with_indifferent_access
       @prefix_options = {}
+      @schema         = self.class.schema_definition.new
       @persisted = persisted
       load(attributes, false, persisted)
     end
@@ -1369,6 +1409,7 @@ def clone
       resource = self.class.new({})
       resource.prefix_options = self.prefix_options
       resource.send :instance_variable_set, "@attributes", cloned
+      resource.send :instance_variable_set, "@schema", @schema.clone
       resource
     end
 
@@ -1702,7 +1743,7 @@ def respond_to_missing?(method, include_priv = false)
       method_name = method.to_s
       if attributes.nil?
         super
-      elsif known_attributes.include?(method_name)
+      elsif known_attributes.include?(method_name) || @schema.respond_to?(method)
         true
       elsif method_name =~ /(?:=|\?)$/ && known_attributes.include?($`)
         true
@@ -1713,6 +1754,10 @@ def respond_to_missing?(method, include_priv = false)
       end
     end
 
+    def serializable_hash(options = nil)
+      @schema.serializable_hash(options).merge!(super)
+    end
+
     def to_json(options = {})
       super(include_root_in_json ? { root: self.class.element_name }.merge(options) : options)
     end
@@ -1733,14 +1778,22 @@ def read_attribute(attr_name)
       name = attr_name.to_s
 
       name = self.class.primary_key if name == "id" && self.class.primary_key
-      @attributes[name]
+      if @schema.respond_to?(name)
+        @schema.send(name)
+      else
+        @attributes[name]
+      end
     end
 
     def write_attribute(attr_name, value)
       name = attr_name.to_s
 
       name = self.class.primary_key if name == "id" && self.class.primary_key
-      @attributes[name] = value
+      if @schema.respond_to?("#{name}=")
+        @schema.send("#{name}=", value)
+      else
+        @attributes[name] = value
+      end
     end
 
     protected
@@ -1882,7 +1935,9 @@ def split_options(options = {})
       def method_missing(method_symbol, *arguments) # :nodoc:
         method_name = method_symbol.to_s
 
-        if method_name =~ /(=|\?)$/
+        if @schema.respond_to?(method_name)
+          @schema.send(method_name, *arguments)
+        elsif method_name =~ /(=|\?)$/
           case $1
           when "="
             write_attribute($`, arguments.first)

lib/active_resource/schema.rb

@@ -2,13 +2,26 @@
 
 module ActiveResource # :nodoc:
   class Schema # :nodoc:
+    include ActiveModel::Model
+    include ActiveModel::Attributes
+    include ActiveModel::Serialization
+
     # attributes can be known to be one of these types. They are easy to
     # cast to/from.
     KNOWN_ATTRIBUTE_TYPES = %w[ string text integer float decimal datetime timestamp time date binary boolean ]
 
     # An array of attribute definitions, representing the attributes that
     # have been defined.
-    attr_accessor :attrs
+    class_attribute :attrs, instance_accessor: false, instance_predicate: false # :nodoc:
+    self.attrs = {}.freeze
+
+    class_attribute :cast_values, instance_accessor: false, instance_predicate: false # :nodoc:
+    self.cast_values = false
+
+    attribute_method_suffix "?", parameters: false
+
+    alias_method :attribute?, :send
+    private :attribute?
 
     # The internals of an Active Resource Schema are very simple -
     # unlike an Active Record TableDefinition (on which it is based).
@@ -22,39 +35,54 @@ class Schema # :nodoc:
     #  The schema stores the name and type of each attribute. That is then
     #  read out by the schema method to populate the schema of the actual
     #  resource.
-    def initialize
-      @attrs = {}
-    end
 
-    def attribute(name, type, options = {})
-      raise ArgumentError, "Unknown Attribute type: #{type.inspect} for key: #{name.inspect}" unless type.nil? || Schema::KNOWN_ATTRIBUTE_TYPES.include?(type.to_s)
+    class << self
+      def inherited(subclass)
+        super
+        subclass.attrs = attrs.dup
+      end
 
-      the_type = type.to_s
-      # TODO: add defaults
-      # the_attr = [type.to_s]
-      # the_attr << options[:default] if options.has_key? :default
-      @attrs[name.to_s] = the_type
-      self
-    end
-
-    # The following are the attribute types supported by Active Resource
-    # migrations.
-    KNOWN_ATTRIBUTE_TYPES.each do |attr_type|
-      # def string(*args)
-      #   options = args.extract_options!
-      #   attr_names = args
+      # The internals of an Active Resource Schema are very simple -
+      # unlike an Active Record TableDefinition (on which it is based).
+      # It provides a set of convenience methods for people to define their
+      # schema using the syntax:
+      #  schema do
+      #    string :foo
+      #    integer :bar
+      #  end
       #
-      #   attr_names.each { |name| attribute(name, 'string', options) }
-      # end
-      class_eval <<-EOV, __FILE__, __LINE__ + 1
-        # frozen_string_literal: true
-        def #{attr_type}(*args)
-          options = args.extract_options!
-          attr_names = args
-
-          attr_names.each { |name| attribute(name, '#{attr_type}', options) }
-        end
-      EOV
+      #  The schema stores the name and type of each attribute. That is then
+      #  read out by the schema method to populate the schema of the actual
+      #  resource.
+      def attribute(name, type = nil, options = {})
+        raise ArgumentError, "Unknown Attribute type: #{type.inspect} for key: #{name.inspect}" unless type.nil? || Schema::KNOWN_ATTRIBUTE_TYPES.include?(type.to_s)
+
+        the_type = type&.to_s
+        attrs[name.to_s] = the_type
+
+        super(name, cast_values ? type.try(:to_sym) : nil, **options)
+        self
+      end
+
+      # The following are the attribute types supported by Active Resource
+      # migrations.
+      KNOWN_ATTRIBUTE_TYPES.each do |attr_type|
+        # def string(*args)
+        #   options = args.extract_options!
+        #   attr_names = args
+        #
+        #   attr_names.each { |name| attribute(name, 'string', options) }
+        # end
+        class_eval <<-EOV, __FILE__, __LINE__ + 1
+          # frozen_string_literal: true
+          def #{attr_type}(*args)
+            options = args.extract_options!
+            attr_names = args
+
+            attr_names.each { |name| attribute(name, '#{attr_type}', options) }
+          end
+        EOV
+      end
     end
   end
 end