fix : fail to alter column from smallint to boolean (#165)

* fix : fail to alter column from smallint to boolean

* fix : fail to alter column from smallint to boolean

* fix : fail to alter column from string to boolean

* fix : fail to alter column from string to boolean if the value is "false" in string

* move using conversion expression to func

* move using expression to func

migrator.go

@@ -315,7 +315,7 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 						return err
 					}
 				} else {
-					if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? TYPE ? USING ?::?",
+					if err := m.DB.Exec("ALTER TABLE ? ALTER COLUMN ? TYPE ?"+m.genUsingExpression(fileType.SQL, fieldColumnType.DatabaseTypeName()),
 						m.CurrentTable(stmt), clause.Column{Name: field.DBName}, fileType, clause.Column{Name: field.DBName}, fileType).Error; err != nil {
 						return err
 					}
@@ -375,6 +375,16 @@ func (m Migrator) AlterColumn(value interface{}, field string) error {
 	return nil
 }
 
+func (m Migrator) genUsingExpression(targetType, sourceType string) string {
+	if targetType == "boolean" {
+		switch sourceType {
+		case "int2", "int8", "numeric":
+			return " USING ?::INT::?"
+		}
+	}
+	return " USING ?::?"
+}
+
 func (m Migrator) HasConstraint(value interface{}, name string) bool {
 	var count int64
 	m.RunWithValue(value, func(stmt *gorm.Statement) error {