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

3 files changed, 14 insertions, 12 deletions

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