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.

1 files changed, 33 insertions, 0 deletions

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