Adding json type for mssql dialect, similar to postgres.Jsonb (#1934)

* Adding json type for mssql dialect, similar to postgres.Jsonb

* Adding proper comments

dialects/mssql/mssql.go

@@ -1,12 +1,16 @@
 package mssql
 
 import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
 	"strings"
 	"time"
 
+	// Importing mssql driver package only in dialect file, otherwide not needed
 	_ "github.com/denisenkom/go-mssqldb"
 	"github.com/jinzhu/gorm"
 )
@@ -201,3 +205,27 @@ func currentDatabaseAndTable(dialect gorm.Dialect, tableName string) (string, st
 	}
 	return dialect.CurrentDatabase(), tableName
 }
+
+// JSON type to support easy handling of JSON data in character table fields
+// using golang json.RawMessage for deferred decoding/encoding
+type JSON struct {
+	json.RawMessage
+}
+
+// Value get value of JSON
+func (j JSON) Value() (driver.Value, error) {
+	if len(j.RawMessage) == 0 {
+		return nil, nil
+	}
+	return j.MarshalJSON()
+}
+
+// Scan scan value into JSON
+func (j *JSON) Scan(value interface{}) error {
+	str, ok := value.(string)
+	if !ok {
+		return errors.New(fmt.Sprint("Failed to unmarshal JSONB value (strcast):", value))
+	}
+	bytes := []byte(str)
+	return json.Unmarshal(bytes, j)
+}