datastore: ErrFieldMismatch on sub fields clears out top level array field

[evanwht]

Client

datastore

Environment

MacOS

Go Environment

go1.17.2 darwin/amd64 cloud.google.com/do/datastore v.1.16.0

Code

type (
	ComplexObject struct {
		Field1 string
		Field2 []ComplexSubObject
	}
	ComplexSubObject struct {
		SubField1 string
		SubField2 struct {
			SubSubField1 string
			SubSubField2 string
		}
	}

	SimpleObject struct {
		Field1 string
		Field2 []SimpleSubObject
	}
	SimpleSubObject struct {
		SubField1 string
		SubField2 struct {
			SubSubField1 string
		}
	}
)

func TestFieldMismatchLoading() {
	ctx := context.Background()
	c, err := datastore.NewClient(ctx, "my-project")
	if err != nil {
		panic(err)
	}

	o := ComplexObject{
		Field1: "field-1",
		Field2: []ComplexSubObject{{
			SubField1: "sub-field-1",
			SubField2: struct {
				SubSubField1 string
				SubSubField2 string
			}{
				SubSubField1: "sub-sub-field-1",
				SubSubField2: "sub-sub-field-2",
			},
		}},
	}
	fmt.Printf("%+v\n", o)
	key, err := c.Put(ctx, datastore.IncompleteKey("test", nil), &o)
	if err != nil {
		panic(err)
	}

	var s SimpleObject
	err = c.Get(ctx, key, &s)
	if err != nil {
		if _, ok := err.(*datastore.ErrFieldMismatch); !ok {
			panic(err)
		}
	}
	fmt.Printf("%+v\n", s)
}

Expected behavior

Field2 of the entity should load with one entry that contains an A field and a B that only contains the one field it knows about. The documentation for loading suggests that as many fields are filled out as possible and an error is returned alerting the user that there was a field mismatch.

Actual behavior

Field2 is returned as an empty array.

Additional context

If you change Field2 to not be an array, the sub object loads all possible fields that it can.