Support the AUTO_RANDOM Field (#104)

* feat: support auto_random through default tag

* fix: unsigned type support

* fix: keeping the HasDefaultValue value is true for auto_random

mysql.go

@@ -21,6 +21,10 @@ import (
 	"gorm.io/gorm/utils"
 )
 
+const (
+	AutoRandomTag = "auto_random()" // Treated as an auto_random field for tidb
+)
+
 type Config struct {
 	DriverName                    string
 	ServerVersion                 string
@@ -417,7 +421,36 @@ func (dialector Dialector) getSchemaBytesType(field *schema.Field) string {
 	return "longblob"
 }
 
+// autoRandomType
+// field.DataType MUST be `schema.Int` or `schema.Uint`
+// Judgement logic:
+// 1. Is PrimaryKey;
+// 2. Has default value;
+// 3. Default value is "auto_random()";
+// 4. IGNORE the field.Size, it MUST be bigint;
+// 5. CLEAR the default tag, and return true;
+// 6. Otherwise, return false.
+func autoRandomType(field *schema.Field) (bool, string) {
+	if field.PrimaryKey && field.HasDefaultValue &&
+		strings.ToLower(strings.TrimSpace(field.DefaultValue)) == AutoRandomTag {
+		field.DefaultValue = ""
+
+		sqlType := "bigint"
+		if field.DataType == schema.Uint {
+			sqlType += " unsigned"
+		}
+		sqlType += " auto_random"
+		return true, sqlType
+	}
+
+	return false, ""
+}
+
 func (dialector Dialector) getSchemaIntAndUnitType(field *schema.Field) string {
+	if autoRandom, typeString := autoRandomType(field); autoRandom {
+		return typeString
+	}
+
 	constraint := func(sqlType string) string {
 		if field.DataType == schema.Uint {
 			sqlType += " unsigned"