Validate keys in the `_fixture` field

Otherwise it can be very hard to figure out that we made a typo.

Fix: https://github.com/rails/rails/issues/42441

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Fixture configurations (`_fixture`) are now strictly validated.
+
+    If an error will be raised if that entry contains unknown keys while previously it
+    would silently have no effects.
+
+    *Jean Boussier*
+
 *   Add `ActiveRecord::Base.update!` that works like `ActiveRecord::Base.update` but raises exceptions.
 
     This allows for the same behavior as the instance method `#update!` at a class level.

activerecord/lib/active_record/fixture_set/file.rb

@@ -41,7 +41,7 @@ module ActiveRecord
           @config_row ||= begin
             row = raw_rows.find { |fixture_name, _| fixture_name == "_fixture" }
             if row
-              row.last
+              validate_config_row(row.last)
             else
               { 'model_class': nil, 'ignore': nil }
             end
@@ -58,6 +58,20 @@ module ActiveRecord
           end
         end
 
+        def validate_config_row(data)
+          unless Hash === data
+            raise Fixture::FormatError, "Invalid `_fixture` section: `_fixture` must be a hash: #{@file}"
+          end
+
+          begin
+            data.assert_valid_keys("model_class", "ignore")
+          rescue ArgumentError => error
+            raise Fixture::FormatError, "Invalid `_fixture` section: #{error.message}: #{@file}"
+          end
+
+          data
+        end
+
         # Validate our unmarshalled data.
         def validate(data)
           unless Hash === data || YAML::Omap === data

activerecord/test/cases/fixture_set/file_test.rb

@@ -70,6 +70,15 @@ module ActiveRecord
         end
       end
 
+      def test_wrong_config_row
+        tmp_yaml ["empty", "yml"], { "_fixture" => { "class_name" => "Foo" } }.to_yaml do |t|
+          error = assert_raises(ActiveRecord::Fixture::FormatError) do
+            File.open(t.path) { |fh| fh.model_class }
+          end
+          assert_includes error.message, "Invalid `_fixture` section"
+        end
+      end
+
       def test_render_context_helper
         ActiveRecord::FixtureSet.context_class.class_eval do
           def fixture_helper