Return a null column from `column_for_attribute` when no column exists.

This reverts commit ae96f229f6501d8635811d6b22d75d43cdb880a4.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/attribute_methods.rb

5 files changed, 36 insertions, 16 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 17e2c22a38..34b7502d3d 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Return a null column from `column_for_attribute` when no column exists.
+
+    *Rafael Mendonça França*
+
 *   Remove deprecated `serialized_attributes`.
 
     *Rafael Mendonça França*
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index a6e1a24360..afcaa5d55a 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -104,11 +104,11 @@ module ActiveRecord
         end
 
         def association_key_type
-          @klass.type_for_attribute(association_key_name.to_s).type
+          @klass.column_for_attribute(association_key_name).type
         end
 
         def owner_key_type
-          @model.type_for_attribute(owner_key_name.to_s).type
+          @model.column_for_attribute(owner_key_name).type
         end
 
         def load_slices(slices)
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index b7edac791e..8f165fb1dc 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -192,7 +192,8 @@ module ActiveRecord
       end
 
       # Returns the column object for the named attribute.
-      # Returns nil if the named attribute does not exist.
+      # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
+      # named attribute does not exist.
       #
       #   class Person < ActiveRecord::Base
       #   end
@@ -202,17 +203,12 @@ module ActiveRecord
       #   # => #<ActiveRecord::ConnectionAdapters::Column:0x007ff4ab083980 @name="name", @sql_type="varchar(255)", @null=true, ...>
       #
       #   person.column_for_attribute(:nothing)
-      #   # => nil
+      #   # => #<ActiveRecord::ConnectionAdapters::NullColumn:0xXXX @name=nil, @sql_type=nil, @cast_type=#<Type::Value>, ...>
       def column_for_attribute(name)
-        column = columns_hash[name.to_s]
-        if column.nil?
-          ActiveSupport::Deprecation.warn(<<-MSG.squish)
-            `#column_for_attribute` will return a null object for non-existent
-            columns in Rails 5. Use `#has_attribute?` if you need to check for
-            an attribute's existence.
-          MSG
+        name = name.to_s
+        columns_hash.fetch(name) do
+          ConnectionAdapters::NullColumn.new(name)
         end
-        column
       end
     end
 
diff --git a/activerecord/lib/active_record/connection_adapters/column.rb b/activerecord/lib/active_record/connection_adapters/column.rb
index 65d8b1a8ab..d7f999c3c6 100644
--- a/activerecord/lib/active_record/connection_adapters/column.rb
+++ b/activerecord/lib/active_record/connection_adapters/column.rb
@@ -77,6 +77,12 @@ module ActiveRecord
         [self.class, name, default, cast_type, sql_type, null, default_function]
       end
     end
+
+    class NullColumn < Column
+      def initialize(name)
+        super name, nil, Type::Value.new
+      end
+    end
   end
   # :startdoc:
 end
diff --git a/activerecord/test/cases/reflection_test.rb b/activerecord/test/cases/reflection_test.rb
index e86b892a0a..a2252a836f 100644
--- a/activerecord/test/cases/reflection_test.rb
+++ b/activerecord/test/cases/reflection_test.rb
@@ -80,10 +80,24 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal :integer, @first.column_for_attribute("id").type
   end
 
-  def test_non_existent_columns_return_nil
-    assert_deprecated do
-      assert_nil @first.column_for_attribute("attribute_that_doesnt_exist")
-    end
+  def test_non_existent_columns_return_null_object
+    column = @first.column_for_attribute("attribute_that_doesnt_exist")
+    assert_instance_of ActiveRecord::ConnectionAdapters::NullColumn, column
+    assert_equal "attribute_that_doesnt_exist", column.name
+    assert_equal nil, column.sql_type
+    assert_equal nil, column.type
+    assert_not column.number?
+    assert_not column.text?
+    assert_not column.binary?
+  end
+
+  def test_non_existent_columns_are_identity_types
+    column = @first.column_for_attribute("attribute_that_doesnt_exist")
+    object = Object.new
+
+    assert_equal object, column.type_cast_from_database(object)
+    assert_equal object, column.type_cast_from_user(object)
+    assert_equal object, column.type_cast_for_database(object)
   end
 
   def test_reflection_klass_for_nested_class_name