1 files changed, 86 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..3a76f5262d
--- /dev/null
+++ b/activerecord/lib/active_record/attribute_set/builder.rb
@@ -0,0 +1,86 @@
+module ActiveRecord
+  class AttributeSet # :nodoc:
+    class Builder # :nodoc:
+      attr_reader :types, :always_initialized
+
+      def initialize(types, always_initialized = nil)
+        @types = types
+        @always_initialized = always_initialized
+      end
+
+      def build_from_database(values = {}, additional_types = {})
+        if always_initialized && !values.key?(always_initialized)
+          values[always_initialized] = nil
+        end
+
+        attributes = LazyAttributeHash.new(types, values, additional_types)
+        AttributeSet.new(attributes)
+      end
+    end
+  end
+
+  class LazyAttributeHash # :nodoc:
+    delegate :select, :transform_values, to: :materialize
+
+    def initialize(types, values, additional_types)
+      @types = types
+      @values = values
+      @additional_types = additional_types
+      @materialized = false
+      @delegate_hash = {}
+    end
+
+    def key?(key)
+      delegate_hash.key?(key) || values.key?(key) || types.key?(key)
+    end
+
+    def [](key)
+      delegate_hash[key] || assign_default_value(key)
+    end
+
+    def []=(key, value)
+      if frozen?
+        raise RuntimeError, "Can't modify frozen hash"
+      end
+      delegate_hash[key] = value
+    end
+
+    def initialized_keys
+      delegate_hash.keys | values.keys
+    end
+
+    def initialize_dup(_)
+      @delegate_hash = delegate_hash.transform_values(&:dup)
+      super
+    end
+
+    protected
+
+    attr_reader :types, :values, :additional_types, :delegate_hash
+
+    private
+
+    def assign_default_value(name)
+      type = additional_types.fetch(name, types[name])
+      value_present = true
+      value = values.fetch(name) { value_present = false }
+
+      if value_present
+        delegate_hash[name] = Attribute.from_database(name, value, type)
+      elsif types.key?(name)
+        delegate_hash[name] = Attribute.uninitialized(name, type)
+      end
+    end
+
+    def materialize
+      unless @materialized
+        values.each_key { |key| self[key] }
+        types.each_key { |key| self[key] }
+        unless frozen?
+          @materialized = true
+        end
+      end
+      delegate_hash
+    end
+  end
+end