Added JSON support for NullDecimal (#62)

* Corrected NullDecimal comment

Saying that NullDecimal is immutable is just incorrect, and we don't need
to duplicate the description of properties of a Decimal type here.

* Added UnmarshalJSON and MarshalJSON support for NullDecimal

* Added tests for JSON operations on NullDecimal

* Fixed mistakes in tests

2 files changed, 112 insertions, 2 deletions

diff --git a/decimal.go b/decimal.go
index 1729b61..6078361 100644
--- a/decimal.go
+++ b/decimal.go
@@ -928,8 +928,8 @@ func unquoteIfQuoted(value interface{}) (string, error) {
 	return string(bytes), nil
 }
 
-// NullDecimal represents a fixed-point decimal. It is immutable.
-// number = value * 10 ^ exp
+// NullDecimal represents a nullable decimal with compatibility for
+// scanning null values from the database.
 type NullDecimal struct {
 	Decimal Decimal
 	Valid   bool
@@ -952,3 +952,21 @@ func (d NullDecimal) Value() (driver.Value, error) {
 	}
 	return d.Decimal.Value()
 }
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (d *NullDecimal) UnmarshalJSON(decimalBytes []byte) error {
+	if string(decimalBytes) == "null" {
+		d.Valid = false
+		return nil
+	}
+	d.Valid = true
+	return d.Decimal.UnmarshalJSON(decimalBytes)
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (d NullDecimal) MarshalJSON() ([]byte, error) {
+	if !d.Valid {
+		return []byte("null"), nil
+	}
+	return d.Decimal.MarshalJSON()
+}
diff --git a/decimal_test.go b/decimal_test.go
index d606dc9..9786bcc 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -332,6 +332,98 @@ func TestBadJSON(t *testing.T) {
 	}
 }
 
+func TestNullDecimalJSON(t *testing.T) {
+	for _, s := range testTable {
+		var doc struct {
+			Amount NullDecimal `json:"amount"`
+		}
+		docStr := `{"amount":"` + s + `"}`
+		docStrNumber := `{"amount":` + s + `}`
+		err := json.Unmarshal([]byte(docStr), &doc)
+		if err != nil {
+			t.Errorf("error unmarshaling %s: %v", docStr, err)
+		} else {
+			if !doc.Amount.Valid {
+				t.Errorf("expected %s to be valid (not NULL), got Valid = false", s)
+			}
+			if doc.Amount.Decimal.String() != s {
+				t.Errorf("expected %s, got %s (%s, %d)",
+					s, doc.Amount.Decimal.String(),
+					doc.Amount.Decimal.value.String(), doc.Amount.Decimal.exp)
+			}
+		}
+
+		out, err := json.Marshal(&doc)
+		if err != nil {
+			t.Errorf("error marshaling %+v: %v", doc, err)
+		} else if string(out) != docStr {
+			t.Errorf("expected %s, got %s", docStr, string(out))
+		}
+
+		// make sure unquoted marshalling works too
+		MarshalJSONWithoutQuotes = true
+		out, err = json.Marshal(&doc)
+		if err != nil {
+			t.Errorf("error marshaling %+v: %v", doc, err)
+		} else if string(out) != docStrNumber {
+			t.Errorf("expected %s, got %s", docStrNumber, string(out))
+		}
+		MarshalJSONWithoutQuotes = false
+	}
+
+	var doc struct {
+		Amount NullDecimal `json:"amount"`
+	}
+	docStr := `{"amount": null}`
+	err := json.Unmarshal([]byte(docStr), &doc)
+	if err != nil {
+		t.Errorf("error unmarshaling %s: %v", docStr, err)
+	} else if doc.Amount.Valid {
+		t.Errorf("expected null value to have Valid = false, got Valid = true and Decimal = %s (%s, %d)",
+			doc.Amount.Decimal.String(),
+			doc.Amount.Decimal.value.String(), doc.Amount.Decimal.exp)
+	}
+
+	expected := `{"amount":null}`
+	out, err := json.Marshal(&doc)
+	if err != nil {
+		t.Errorf("error marshaling %+v: %v", doc, err)
+	} else if string(out) != expected {
+		t.Errorf("expected %s, got %s", expected, string(out))
+	}
+
+	// make sure unquoted marshalling works too
+	MarshalJSONWithoutQuotes = true
+	expectedUnquoted := `{"amount":null}`
+	out, err = json.Marshal(&doc)
+	if err != nil {
+		t.Errorf("error marshaling %+v: %v", doc, err)
+	} else if string(out) != expectedUnquoted {
+		t.Errorf("expected %s, got %s", expectedUnquoted, string(out))
+	}
+	MarshalJSONWithoutQuotes = false
+}
+
+func TestNullDecimalBadJSON(t *testing.T) {
+	for _, testCase := range []string{
+		"]o_o[",
+		"{",
+		`{"amount":""`,
+		`{"amount":""}`,
+		`{"amount":"nope"}`,
+		`{"amount":nope}`,
+		`0.333`,
+	} {
+		var doc struct {
+			Amount NullDecimal `json:"amount"`
+		}
+		err := json.Unmarshal([]byte(testCase), &doc)
+		if err == nil {
+			t.Errorf("expected error, got %+v", doc)
+		}
+	}
+}
+
 func TestXML(t *testing.T) {
 	for _, s := range testTable {
 		var doc struct {