Use db tag to customize ID field (#453)

This will only work with Find method for now, but we can migrate other hard-coded IDs after.

model.go

@@ -42,6 +42,20 @@ func (m *Model) ID() interface{} {
 	return fbn.Interface()
 }
 
+// IDField returns the name of the DB field used for the ID.
+// By default, it will return "id".
+func (m *Model) IDField() string {
+	field, ok := reflect.TypeOf(m.Value).Elem().FieldByName("ID")
+	if !ok {
+		return "id"
+	}
+	dbField := field.Tag.Get("db")
+	if dbField == "" {
+		return "id"
+	}
+	return dbField
+}
+
 // PrimaryKeyType gives the primary key type of the `Model`.
 func (m *Model) PrimaryKeyType() string {
 	fbn, err := m.fieldByName("ID")
@@ -174,11 +188,11 @@ func (m *Model) touchUpdatedAt() {
 }
 
 func (m *Model) whereID() string {
-	return fmt.Sprintf("%s.id = ?", m.TableName())
+	return fmt.Sprintf("%s.%s = ?", m.TableName(), m.IDField())
 }
 
 func (m *Model) whereNamedID() string {
-	return fmt.Sprintf("%s.id = :id", m.TableName())
+	return fmt.Sprintf("%s.%s = :id", m.TableName(), m.IDField())
 }
 
 func (m *Model) isSlice() bool {

model_test.go

@@ -130,3 +130,19 @@ func Test_Touch_Unix_Timestamp_With_Existing_Value(t *testing.T) {
 	r.Equal(createdAt, v.CreatedAt)
 	r.Equal(int(t0.Unix()), v.UpdatedAt)
 }
+
+func Test_IDField(t *testing.T) {
+	r := require.New(t)
+
+	type testCustomID struct {
+		ID int `db:"custom_id"`
+	}
+	m := Model{Value: &testCustomID{ID: 1}}
+	r.Equal("custom_id", m.IDField())
+
+	type testNormalID struct {
+		ID int
+	}
+	m = Model{Value: &testNormalID{ID: 1}}
+	r.Equal("id", m.IDField())
+}