chore: improve null merging behaviour

mergevalues.go

@@ -18,15 +18,17 @@ func MergeValues(a, b *Value) (v *Value, changed bool, err error) {
 	if b == nil {
 		return a, false, nil
 	}
+	if b.Type() == TypeNull && a.Type() == TypeObject {
+		// we assume that null was returned in an error case for resolving a nested object field
+		// as we've got an object on the left side, we don't override the whole object with null
+		// instead, we keep the left object and discard the null on the right side
+		return a, false, nil
+	}
 	aBool, bBool := a.Type() == TypeTrue || a.Type() == TypeFalse, b.Type() == TypeTrue || b.Type() == TypeFalse
 	booleans := aBool && bBool
-	oneIsNull := a.Type() == TypeNull || b.Type() == TypeNull
-	if a.Type() != b.Type() && !booleans && !oneIsNull {
+	if a.Type() != b.Type() && !booleans {
 		return nil, false, ErrMergeDifferentTypes
 	}
-	if b.Type() == TypeNull && a.Type() != TypeNull {
-		return b, true, nil
-	}
 	switch a.Type() {
 	case TypeObject:
 		ao, _ := a.Object()

mergevalues_test.go

@@ -183,20 +183,14 @@ func TestMergeValues(t *testing.T) {
 	t.Run("null not null", func(t *testing.T) {
 		t.Parallel()
 		a, b := MustParse(`null`), MustParse(`1`)
-		merged, changed, err := MergeValues(a, b)
-		require.NoError(t, err)
-		require.Equal(t, true, changed)
-		out := merged.MarshalTo(nil)
-		require.Equal(t, `1`, string(out))
+		_, _, err := MergeValues(a, b)
+		require.Error(t, err)
 	})
 	t.Run("null not null reverse", func(t *testing.T) {
 		t.Parallel()
 		a, b := MustParse(`1`), MustParse(`null`)
-		merged, changed, err := MergeValues(a, b)
-		require.NoError(t, err)
-		require.Equal(t, true, changed)
-		out := merged.MarshalTo(nil)
-		require.Equal(t, `null`, string(out))
+		_, _, err := MergeValues(a, b)
+		require.Error(t, err)
 	})
 	t.Run("array objects", func(t *testing.T) {
 		t.Parallel()
@@ -272,32 +266,43 @@ func TestMergeValues(t *testing.T) {
 		t.Parallel()
 		left := MustParse(`null`)
 		right := MustParse(`true`)
-		out, _, err := MergeValues(left, right)
-		require.NoError(t, err)
-		require.Equal(t, `true`, out.String())
+		_, _, err := MergeValues(left, right)
+		require.Error(t, err)
 	})
 	t.Run("true null", func(t *testing.T) {
 		t.Parallel()
 		left := MustParse(`true`)
 		right := MustParse(`null`)
-		out, _, err := MergeValues(left, right)
-		require.NoError(t, err)
-		require.Equal(t, `null`, out.String())
+		_, _, err := MergeValues(left, right)
+		require.Error(t, err)
 	})
 	t.Run("nested null true", func(t *testing.T) {
 		t.Parallel()
 		left := MustParse(`{"a":null}`)
 		right := MustParse(`{"a":true}`)
-		out, _, err := MergeValues(left, right)
-		require.NoError(t, err)
-		require.Equal(t, `{"a":true}`, out.String())
+		_, _, err := MergeValues(left, right)
+		require.Error(t, err)
 	})
 	t.Run("nested true null", func(t *testing.T) {
 		t.Parallel()
 		left := MustParse(`{"a":true}`)
 		right := MustParse(`{"a":null}`)
+		_, _, err := MergeValues(left, right)
+		require.Error(t, err)
+	})
+	t.Run("nested null into nested object", func(t *testing.T) {
+		t.Parallel()
+		left := MustParse(`{"a":{"b":"c"}}`)
+		right := MustParse(`{"a":null}`)
 		out, _, err := MergeValues(left, right)
 		require.NoError(t, err)
-		require.Equal(t, `{"a":null}`, out.String())
+		require.Equal(t, `{"a":{"b":"c"}}`, out.String())
+	})
+	t.Run("nested object into nested null", func(t *testing.T) {
+		t.Parallel()
+		left := MustParse(`{"a":null}`)
+		right := MustParse(`{"a":{"b":"c"}}`)
+		_, _, err := MergeValues(left, right)
+		require.Error(t, err)
 	})
 }