Move behavior of `read_attribute` to `AttributeSet`

Moved `Builder` to its own file, as it started looking very weird once I
added private methods to the `AttributeSet` class and the `Builder`
class started to grow.

Would like to refactor `fetch_value` to change to

```ruby
self[name].value(&block)
```

But that requires the attributes to know about their name, which they
currently do not.

5 files changed, 88 insertions, 28 deletions

diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index ab85414277..17b00bbaea 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -27,12 +27,12 @@ require 'active_model'
 require 'arel'
 
 require 'active_record/version'
+require 'active_record/attribute_set'
 
 module ActiveRecord
   extend ActiveSupport::Autoload
 
   autoload :Attribute
-  autoload :AttributeSet
   autoload :Base
   autoload :Callbacks
   autoload :Core
diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
index da8eb10dc6..6c0b81b3fe 100644
--- a/activerecord/lib/active_record/attribute.rb
+++ b/activerecord/lib/active_record/attribute.rb
@@ -8,6 +8,10 @@ module ActiveRecord
       def from_user(value, type)
         FromUser.new(value, type)
       end
+
+      def uninitialized(type)
+        Uninitialized.new(type)
+      end
     end
 
     attr_reader :value_before_type_cast, :type
@@ -41,6 +45,10 @@ module ActiveRecord
       raise NotImplementedError
     end
 
+    def initialized?
+      true
+    end
+
     protected
 
     def initialize_dup(other)
@@ -69,6 +77,25 @@ module ActiveRecord
           false
         end
         alias changed_in_place_from? changed_from?
+
+        def initialized?
+          true
+        end
+      end
+    end
+
+    class Uninitialized < Attribute # :nodoc:
+      def initialize(type)
+        super(nil, type)
+      end
+
+      def value
+        nil
+      end
+      alias value_for_database value
+
+      def initialized?
+        false
       end
     end
   end
diff --git a/activerecord/lib/active_record/attribute_methods/read.rb b/activerecord/lib/active_record/attribute_methods/read.rb
index 8c1cc128f7..10869dfc1e 100644
--- a/activerecord/lib/active_record/attribute_methods/read.rb
+++ b/activerecord/lib/active_record/attribute_methods/read.rb
@@ -81,17 +81,10 @@ module ActiveRecord
       # Returns the value of the attribute identified by <tt>attr_name</tt> after
       # it has been typecast (for example, "2004-12-12" in a date column is cast
       # to a date object, like Date.new(2004, 12, 12)).
-      def read_attribute(attr_name)
+      def read_attribute(attr_name, &block)
         name = attr_name.to_s
-        @attributes.fetch(name) {
-          if name == 'id'
-            return read_attribute(self.class.primary_key)
-          elsif block_given? && self.class.columns_hash.key?(name)
-            return yield(name)
-          else
-            return nil
-          end
-        }.value
+        name = self.class.primary_key if name == 'id'
+        @attributes.fetch_value(name, &block)
       end
 
       private
diff --git a/activerecord/lib/active_record/attribute_set.rb b/activerecord/lib/active_record/attribute_set.rb
index ed2500a675..228e82f332 100644
--- a/activerecord/lib/active_record/attribute_set.rb
+++ b/activerecord/lib/active_record/attribute_set.rb
@@ -1,16 +1,32 @@
+require 'active_record/attribute_set/builder'
+
 module ActiveRecord
   class AttributeSet # :nodoc:
-    delegate :[], :[]=, :fetch, :include?, :keys, :each_with_object, to: :attributes
+    delegate :[], :[]=, :each_with_object, to: :attributes
+    delegate :keys, to: :initialized_attributes
 
     def initialize(attributes)
       @attributes = attributes
     end
 
     def to_hash
-      attributes.each_with_object({}) { |(k, v), h| h[k] = v.value }
+      initialized_attributes.each_with_object({}) { |(k, v), h| h[k] = v.value }
     end
     alias_method :to_h, :to_hash
 
+    def include?(name)
+      attributes.include?(name) && self[name].initialized?
+    end
+
+    def fetch_value(name)
+      attribute = self[name]
+      if attribute.initialized? || !block_given?
+        attribute.value
+      else
+        yield name
+      end
+    end
+
     def freeze
       @attributes.freeze
       super
@@ -30,23 +46,14 @@ module ActiveRecord
       super
     end
 
-    class Builder # :nodoc:
-      def initialize(types)
-        @types = types
-      end
-
-      def build_from_database(values, additional_types = {})
-        attributes = Hash.new(Attribute::Null)
-        values.each_with_object(attributes) do |(name, value), hash|
-          type = additional_types.fetch(name, @types[name])
-          hash[name] = Attribute.from_database(value, type)
-        end
-        AttributeSet.new(attributes)
-      end
-    end
-
     protected
 
     attr_reader :attributes
+
+    private
+
+    def initialized_attributes
+      attributes.select { |_, attr| attr.initialized? }
+    end
   end
 end
diff --git a/activerecord/lib/active_record/attribute_set/builder.rb b/activerecord/lib/active_record/attribute_set/builder.rb
new file mode 100644
index 0000000000..d91720d5e1
--- /dev/null
+++ b/activerecord/lib/active_record/attribute_set/builder.rb
@@ -0,0 +1,33 @@
+module ActiveRecord
+  class AttributeSet # :nodoc:
+    class Builder # :nodoc:
+      attr_reader :types
+
+      def initialize(types)
+        @types = types
+      end
+
+      def build_from_database(values, additional_types = {})
+        attributes = build_attributes_from_values(values, additional_types)
+        add_uninitialized_attributes(attributes)
+        AttributeSet.new(attributes)
+      end
+
+      private
+
+      def build_attributes_from_values(values, additional_types)
+        attributes = Hash.new(Attribute::Null)
+        values.each_with_object(attributes) do |(name, value), hash|
+          type = additional_types.fetch(name, types[name])
+          hash[name] = Attribute.from_database(value, type)
+        end
+      end
+
+      def add_uninitialized_attributes(attributes)
+        types.except(*attributes.keys).each do |name, type|
+          attributes[name] = Attribute.uninitialized(type)
+        end
+      end
+    end
+  end
+end