diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 9bd333bbac..104d84a1f8 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -120,6 +120,7 @@ module ActiveRecord
         def primary_key=(value)
           @primary_key        = value && value.to_s
           @quoted_primary_key = nil
+          @attributes_builder = nil
         end
       end
     end
diff --git a/activerecord/lib/active_record/attribute_set.rb b/activerecord/lib/active_record/attribute_set.rb
index 98ac63c7e1..21c58cbf1d 100644
--- a/activerecord/lib/active_record/attribute_set.rb
+++ b/activerecord/lib/active_record/attribute_set.rb
@@ -2,8 +2,6 @@ require 'active_record/attribute_set/builder'
 
 module ActiveRecord
   class AttributeSet # :nodoc:
-    delegate :keys, to: :initialized_attributes
-
     def initialize(attributes)
       @attributes = attributes
     end
@@ -25,6 +23,10 @@ module ActiveRecord
       attributes.key?(name) && self[name].initialized?
     end
 
+    def keys
+      attributes.initialized_keys
+    end
+
     def fetch_value(name, &block)
       self[name].value(&block)
     end
@@ -43,7 +45,7 @@ module ActiveRecord
     end
 
     def initialize_dup(_)
-      @attributes = attributes.transform_values(&:dup)
+      @attributes = attributes.dup
       super
     end
 
@@ -58,12 +60,6 @@ module ActiveRecord
       end
     end
 
-    def ensure_initialized(key)
-      unless self[key].initialized?
-        write_from_database(key, nil)
-      end
-    end
-
     protected
 
     attr_reader :attributes
diff --git a/activerecord/lib/active_record/attribute_set/builder.rb b/activerecord/lib/active_record/attribute_set/builder.rb
index d4a787f2fe..3946e02d10 100644
--- a/activerecord/lib/active_record/attribute_set/builder.rb
+++ b/activerecord/lib/active_record/attribute_set/builder.rb
@@ -1,35 +1,83 @@
 module ActiveRecord
   class AttributeSet # :nodoc:
     class Builder # :nodoc:
-      attr_reader :types
+      attr_reader :types, :always_initialized
 
-      def initialize(types)
+      def initialize(types, always_initialized = nil)
         @types = types
+        @always_initialized = always_initialized
       end
 
       def build_from_database(values = {}, additional_types = {})
-        attributes = build_attributes_from_values(values, additional_types)
-        add_uninitialized_attributes(attributes)
+        if always_initialized && !values.key?(always_initialized)
+          values[always_initialized] = nil
+        end
+
+        attributes = LazyAttributeHash.new(types, values, additional_types)
         AttributeSet.new(attributes)
       end
 
       private
+    end
+  end
 
-      def build_attributes_from_values(values, additional_types)
-        values.each_with_object({}) do |(name, value), hash|
-          type = additional_types.fetch(name, types[name])
-          hash[name] = Attribute.from_database(name, value, type)
-        end
-      end
+  class LazyAttributeHash
+    delegate :select, :transform_values, to: :materialize
+    delegate :[], :[]=, :freeze, to: :delegate_hash
 
-      def add_uninitialized_attributes(attributes)
-        types.each_key do |name|
-          next if attributes.key? name
-          type = types[name]
-          attributes[name] =
-            Attribute.uninitialized(name, type)
+    def initialize(types, values, additional_types)
+      @types = types
+      @values = values
+      @additional_types = additional_types
+      @materialized = false
+      @delegate_hash = {}
+      assign_default_proc
+    end
+
+    def key?(key)
+      delegate_hash.key?(key) || values.key?(key) || types.key?(key)
+    end
+
+    def initialized_keys
+      delegate_hash.keys | values.keys
+    end
+
+    def initialize_dup(_)
+      @delegate_hash = delegate_hash.transform_values(&:dup)
+      assign_default_proc
+      super
+    end
+
+    def initialize_clone(_)
+      @delegate_hash = delegate_hash.clone
+      super
+    end
+
+    protected
+
+    attr_reader :types, :values, :additional_types, :delegate_hash
+
+    private
+
+    def assign_default_proc
+      delegate_hash.default_proc = proc do |hash, name|
+        type = additional_types.fetch(name, types[name])
+
+        if values.key?(name)
+          hash[name] = Attribute.from_database(name, values[name], type)
+        elsif type
+          hash[name] = Attribute.uninitialized(name, type)
         end
       end
     end
+
+    def materialize
+      unless @materialized
+        values.each_key { |key| delegate_hash[key] }
+        types.each_key { |key| delegate_hash[key] }
+        @materialized = true
+      end
+      delegate_hash
+    end
   end
 end
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 952aeaa703..89d8932e9e 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -536,8 +536,6 @@ module ActiveRecord
     end
 
     def init_internals
-      @attributes.ensure_initialized(self.class.primary_key)
-
       @aggregation_cache        = {}
       @association_cache        = {}
       @readonly                 = false
diff --git a/activerecord/lib/active_record/model_schema.rb b/activerecord/lib/active_record/model_schema.rb
index a444aac23c..adad7774b9 100644
--- a/activerecord/lib/active_record/model_schema.rb
+++ b/activerecord/lib/active_record/model_schema.rb
@@ -231,7 +231,7 @@ module ActiveRecord
       end
 
       def attributes_builder # :nodoc:
-        @attributes_builder ||= AttributeSet::Builder.new(column_types)
+        @attributes_builder ||= AttributeSet::Builder.new(column_types, primary_key)
       end
 
       def column_types # :nodoc:
diff --git a/activerecord/test/cases/attribute_set_test.rb b/activerecord/test/cases/attribute_set_test.rb
index dc20c3c676..a531be482e 100644
--- a/activerecord/test/cases/attribute_set_test.rb
+++ b/activerecord/test/cases/attribute_set_test.rb
@@ -123,6 +123,15 @@ module ActiveRecord
       assert_nil attributes.fetch_value(:bar)
     end
 
+    test "the primary_key is always initialized" do
+      builder = AttributeSet::Builder.new({ foo: Type::Integer.new }, :foo)
+      attributes = builder.build_from_database
+
+      assert attributes.key?(:foo)
+      assert_equal [:foo], attributes.keys
+      assert attributes[:foo].initialized?
+    end
+
     class MyType
       def type_cast_from_user(value)
         return if value.nil?