Added NullDecimal. Changed Decimal to implement driver.Valuer.

From comments in PR 16.

decimal.go

@@ -521,10 +521,7 @@ func (d *Decimal) Scan(value interface{}) error {
 }
 
 // Value implements the driver.Valuer interface for database serialization.
-func (d *Decimal) Value() (driver.Value, error) {
-	if d == nil {
-		return nil, nil
-	}
+func (d Decimal) Value() (driver.Value, error) {
 	return d.String(), nil
 }
 
@@ -672,3 +669,28 @@ func unquoteIfQuoted(value interface{}) (string, error) {
 	}
 	return string(bytes), nil
 }
+
+// NullDecimal represents a fixed-point decimal. It is immutable.
+// number = value * 10 ^ exp
+type NullDecimal struct {
+	Decimal Decimal
+	Valid   bool
+}
+
+// Scan implements the sql.Scanner interface for database deserialization.
+func (d *NullDecimal) Scan(value interface{}) error {
+	if value == nil {
+		d.Valid = false
+		return nil
+	}
+	d.Valid = true
+	return d.Decimal.Scan(value)
+}
+
+// Value implements the driver.Valuer interface for database serialization.
+func (d NullDecimal) Value() (driver.Value, error) {
+	if !d.Valid {
+		return nil, nil
+	}
+	return d.Decimal.String(), nil
+}

decimal_test.go

@@ -1,6 +1,7 @@
 package decimal
 
 import (
+	"database/sql/driver"
 	"encoding/json"
 	"encoding/xml"
 	"math"
@@ -941,19 +942,16 @@ func TestDecimal_Scan(t *testing.T) {
 }
 
 func TestDecimal_Value(t *testing.T) {
-	// check that nil is handled appropriately
-	var decimalPtr *Decimal
-	value, err := decimalPtr.Value()
-	if err != nil {
-		t.Errorf("(*Decimal)(<nil>).Value() failed with message: %s", err)
-	} else if value != nil {
-		t.Errorf("%v is not nil", value)
+	// Make sure this does implement the database/sql's driver.Valuer interface
+	var d Decimal
+	if _, ok := interface{}(d).(driver.Valuer); !ok {
+		t.Error("Decimal does not implement driver.Valuer")
 	}
 
 	// check that normal case is handled appropriately
 	a := New(1234, -2)
 	expected := "12.34"
-	value, err = a.Value()
+	value, err := a.Value()
 	if err != nil {
 		t.Errorf("Decimal(12.34).Value() failed with message: %s", err)
 	} else if value.(string) != expected {
@@ -1059,3 +1057,131 @@ func Benchmark_Cmp(b *testing.B) {
 		sort.Sort(decimals)
 	}
 }
+
+func TestNullDecimal_Scan(t *testing.T) {
+	// test the Scan method that implements the
+	// sql.Scanner interface
+	// check for the for different type of values
+	// that are possible to be received from the database
+	// drivers
+
+	// in normal operations the db driver (sqlite at least)
+	// will return an int64 if you specified a numeric format
+
+	// Make sure handles nil values
+	a := NullDecimal{}
+	var dbvaluePtr interface{}
+	err := a.Scan(dbvaluePtr)
+	if err != nil {
+		// Scan failed... no need to test result value
+		t.Errorf("a.Scan(nil) failed with message: %s", err)
+	} else {
+		if a.Valid {
+			t.Errorf("%s is not null", a.Decimal)
+		}
+	}
+
+	dbvalue := float64(54.33)
+	expected := NewFromFloat(dbvalue)
+
+	err = a.Scan(dbvalue)
+	if err != nil {
+		// Scan failed... no need to test result value
+		t.Errorf("a.Scan(54.33) failed with message: %s", err)
+
+	} else {
+		// Scan suceeded... test resulting values
+		if !a.Valid {
+			t.Errorf("%s is null", a.Decimal)
+		} else if !a.Decimal.Equals(expected) {
+			t.Errorf("%s does not equal to %s", a.Decimal, expected)
+		}
+	}
+
+	// at least SQLite returns an int64 when 0 is stored in the db
+	// and you specified a numeric format on the schema
+	dbvalueInt := int64(0)
+	expected = New(dbvalueInt, 0)
+
+	err = a.Scan(dbvalueInt)
+	if err != nil {
+		// Scan failed... no need to test result value
+		t.Errorf("a.Scan(0) failed with message: %s", err)
+
+	} else {
+		// Scan suceeded... test resulting values
+		if !a.Valid {
+			t.Errorf("%s is null", a.Decimal)
+		} else if !a.Decimal.Equals(expected) {
+			t.Errorf("%s does not equal to %s", a, expected)
+		}
+	}
+
+	// in case you specified a varchar in your SQL schema,
+	// the database driver will return byte slice []byte
+	valueStr := "535.666"
+	dbvalueStr := []byte(valueStr)
+	expected, err = NewFromString(valueStr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = a.Scan(dbvalueStr)
+	if err != nil {
+		// Scan failed... no need to test result value
+		t.Errorf("a.Scan('535.666') failed with message: %s", err)
+
+	} else {
+		// Scan suceeded... test resulting values
+		if !a.Valid {
+			t.Errorf("%s is null", a.Decimal)
+		} else if !a.Decimal.Equals(expected) {
+			t.Errorf("%s does not equal to %s", a, expected)
+		}
+	}
+
+	// lib/pq can also return strings
+	expected, err = NewFromString(valueStr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = a.Scan(valueStr)
+	if err != nil {
+		// Scan failed... no need to test result value
+		t.Errorf("a.Scan('535.666') failed with message: %s", err)
+	} else {
+		// Scan suceeded... test resulting values
+		if !a.Valid {
+			t.Errorf("%s is null", a.Decimal)
+		} else if !a.Decimal.Equals(expected) {
+			t.Errorf("%s does not equal to %s", a, expected)
+		}
+	}
+}
+
+func TestNullDecimal_Value(t *testing.T) {
+	// Make sure this does implement the database/sql's driver.Valuer interface
+	var nullDecimal NullDecimal
+	if _, ok := interface{}(nullDecimal).(driver.Valuer); !ok {
+		t.Error("NullDecimal does not implement driver.Valuer")
+	}
+
+	// check that null is handled appropriately
+	value, err := nullDecimal.Value()
+	if err != nil {
+		t.Errorf("NullDecimal{}.Valid() failed with message: %s", err)
+	} else if value != nil {
+		t.Errorf("%v is not nil", value)
+	}
+
+	// check that normal case is handled appropriately
+	a := NullDecimal{Decimal: New(1234, -2), Valid: true}
+	expected := "12.34"
+	value, err = a.Value()
+	if err != nil {
+		t.Errorf("NullDecimal(12.34).Value() failed with message: %s", err)
+	} else if value.(string) != expected {
+		t.Errorf("%s does not equal to %s", a, expected)
+	}
+}