Simplify creation of default attributes on AR instance

`AttributeSet#dup` has all the behavior we need.

activerecord/lib/active_record/attributes.rb

@@ -110,13 +110,12 @@ module ActiveRecord
 
       def clear_caches_calculated_from_columns
         @attributes_builder = nil
-        @column_defaults = nil
         @column_names = nil
         @column_types = nil
         @columns = nil
         @columns_hash = nil
         @content_columns = nil
-        @raw_column_defaults = nil
+        @default_attributes = nil
       end
     end
   end

activerecord/lib/active_record/base.rb

@@ -9,6 +9,7 @@ require 'active_support/core_ext/class/delegating_attributes'
 require 'active_support/core_ext/array/extract_options'
 require 'active_support/core_ext/hash/deep_merge'
 require 'active_support/core_ext/hash/slice'
+require 'active_support/core_ext/hash/transform_values'
 require 'active_support/core_ext/string/behavior'
 require 'active_support/core_ext/kernel/singleton_class'
 require 'active_support/core_ext/module/introspection'

activerecord/lib/active_record/core.rb

@@ -248,12 +248,7 @@ module ActiveRecord
     #   # Instantiates a single new object
     #   User.new(first_name: 'Jamie')
     def initialize(attributes = nil, options = {})
-      defaults = {}
-      self.class.raw_column_defaults.each do |k, v|
-        defaults[k] = v.duplicable? ? v.dup : v
-      end
-
-      @attributes = self.class.attributes_builder.build_from_database(defaults)
+      @attributes = self.class.default_attributes.dup
 
       init_internals
       initialize_internals_callback

activerecord/lib/active_record/model_schema.rb

@@ -224,8 +224,8 @@ module ActiveRecord
       end
 
       def column_types # :nodoc:
-        @column_types ||= Hash.new(Type::Value.new).tap do |column_types|
-          columns.each { |column| column_types[column.name] = column.cast_type }
+        @column_types ||= columns_hash.transform_values(&:cast_type).tap do |h|
+          h.default = Type::Value.new
         end
       end
 
@@ -236,17 +236,12 @@ module ActiveRecord
       # Returns a hash where the keys are column names and the values are
       # default values when instantiating the AR object for this table.
       def column_defaults
-        @column_defaults ||= Hash[raw_column_defaults.map { |name, default|
-          [name, type_for_attribute(name).type_cast_from_database(default)]
-        }]
+        default_attributes.to_hash
       end
 
-      # Returns a hash where the keys are the column names and the values
-      # are the default values suitable for use in `@raw_attriubtes`
-      def raw_column_defaults # :nodoc:
-        @raw_column_defaults ||= Hash[columns_hash.map { |name, column|
-          [name, column.default]
-        }]
+      def default_attributes # :nodoc:
+        @default_attributes ||= attributes_builder.build_from_database(
+          columns_hash.transform_values(&:default))
       end
 
       # Returns an array of column names as strings.
@@ -292,11 +287,10 @@ module ActiveRecord
         connection.schema_cache.clear_table_cache!(table_name) if table_exists?
 
         @arel_engine             = nil
-        @column_defaults         = nil
-        @raw_column_defaults     = nil
         @column_names            = nil
         @column_types            = nil
         @content_columns         = nil
+        @default_attributes      = nil
         @dynamic_methods_hash    = nil
         @inheritance_column      = nil unless defined?(@explicit_inheritance_column) && @explicit_inheritance_column
         @relation                = nil