Change how `AttributeSet::Builder` receives its defaults

There are two concerns which are both being combined into one here, but
both have the same goal. There are certain attributes which we want to
always consider initialized. Previously, they were handled separately.
The primary key (which is assumed to be backed by a database column)
needs to be initialized, because there is a ton of code in Active Record
that assumes `foo.id` will never raise. Additionally, we want attributes
which aren't backed by a database column to always be initialized, since
we would never receive a database value for them.

Ultimately these two concerns can be combined into one. The old
implementation hid a lot of inherent complexity, and is hard to optimize
from the outside. We can simplify things significantly by just passing
in a hash.

This has slightly different semantics from the old behavior, in that
`Foo.select(:bar).first.id` will return the default value for the
primary key, rather than `nil` unconditionally -- however, the default
value is always `nil` in practice.

activemodel/lib/active_model/attribute_set.rb

@@ -5,7 +5,7 @@ require "active_model/attribute_set/yaml_encoder"
 
 module ActiveModel
   class AttributeSet # :nodoc:
-    delegate :each_value, :fetch, to: :attributes
+    delegate :each_value, :fetch, :except, to: :attributes
 
     def initialize(attributes)
       @attributes = attributes

activemodel/lib/active_model/attribute_set/builder.rb

@@ -5,35 +5,30 @@ require "active_model/attribute"
 module ActiveModel
   class AttributeSet # :nodoc:
     class Builder # :nodoc:
-      attr_reader :types, :always_initialized, :default
+      attr_reader :types, :default_attributes
 
-      def initialize(types, always_initialized = nil, &default)
+      def initialize(types, default_attributes = {})
         @types = types
-        @always_initialized = always_initialized
-        @default = default
+        @default_attributes = default_attributes
       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, &default)
+        attributes = LazyAttributeHash.new(types, values, additional_types, default_attributes)
         AttributeSet.new(attributes)
       end
     end
   end
 
   class LazyAttributeHash # :nodoc:
-    delegate :transform_values, :each_key, :each_value, :fetch, to: :materialize
+    delegate :transform_values, :each_key, :each_value, :fetch, :except, to: :materialize
 
-    def initialize(types, values, additional_types, &default)
+    def initialize(types, values, additional_types, default_attributes)
       @types = types
       @values = values
       @additional_types = additional_types
       @materialized = false
       @delegate_hash = {}
-      @default = default || proc {}
+      @default_attributes = default_attributes
     end
 
     def key?(key)
@@ -94,7 +89,7 @@ module ActiveModel
 
     protected
 
-      attr_reader :types, :values, :additional_types, :delegate_hash, :default
+      attr_reader :types, :values, :additional_types, :delegate_hash, :default_attributes
 
       def materialize
         unless @materialized
@@ -117,7 +112,12 @@ module ActiveModel
         if value_present
           delegate_hash[name] = Attribute.from_database(name, value, type)
         elsif types.key?(name)
-          delegate_hash[name] = default.call(name) || Attribute.uninitialized(name, type)
+          attr = default_attributes[name]
+          if attr
+            delegate_hash[name] = attr.dup
+          else
+            delegate_hash[name] = Attribute.uninitialized(name, type)
+          end
         end
       end
   end

activemodel/test/cases/attribute_set_test.rb

@@ -163,7 +163,8 @@ module ActiveModel
     end
 
     test "the primary_key is always initialized" do
-      builder = AttributeSet::Builder.new({ foo: Type::Integer.new }, :foo)
+      defaults = { foo: Attribute.from_user(:foo, nil, nil) }
+      builder = AttributeSet::Builder.new({ foo: Type::Integer.new }, defaults)
       attributes = builder.build_from_database
 
       assert attributes.key?(:foo)

activerecord/lib/active_record/model_schema.rb

@@ -323,11 +323,11 @@ module ActiveRecord
       end
 
       def attributes_builder # :nodoc:
-        @attributes_builder ||= ActiveModel::AttributeSet::Builder.new(attribute_types, primary_key) do |name|
-          unless columns_hash.key?(name)
-            _default_attributes[name].dup
-          end
+        unless defined?(@attributes_builder) && @attributes_builder
+          defaults = _default_attributes.except(*(column_names - [primary_key]))
+          @attributes_builder = ActiveModel::AttributeSet::Builder.new(attribute_types, defaults)
         end
+        @attributes_builder
       end
 
       def columns_hash # :nodoc: