diff --git a/main.go b/main.go
index bf3e65f8..1ef9c13c 100644
--- a/main.go
+++ b/main.go
@@ -135,13 +135,21 @@ func (s *DB) Assign(attrs ...interface{}) *DB {
 
 func (s *DB) First(out interface{}, where ...interface{}) *DB {
 	scope := s.clone().NewScope(out)
-	scope.Search = scope.Search.clone().order(scope.TableName() + "." + scope.PrimaryKey()).limit(1)
+	if primaryKey := scope.PrimaryKey(); primaryKey != "" {
+		scope.Search = scope.Search.clone().order(scope.TableName() + "." + primaryKey).limit(1)
+	} else {
+		scope.Search = scope.Search.clone().limit(1)
+	}
 	return scope.inlineCondition(where...).callCallbacks(s.parent.callback.queries).db
 }
 
 func (s *DB) Last(out interface{}, where ...interface{}) *DB {
 	scope := s.clone().NewScope(out)
-	scope.Search = scope.Search.clone().order(scope.TableName() + "." + scope.PrimaryKey() + " DESC").limit(1)
+	if primaryKey := scope.PrimaryKey(); primaryKey != "" {
+		scope.Search = scope.Search.clone().order(scope.TableName() + "." + primaryKey + " DESC").limit(1)
+	} else {
+		scope.Search = scope.Search.clone().limit(1)
+	}
 	return scope.inlineCondition(where...).callCallbacks(s.parent.callback.queries).db
 }
 
diff --git a/utils.go b/utils.go
index 04f02a31..ed10e854 100644
--- a/utils.go
+++ b/utils.go
@@ -99,6 +99,7 @@ func GetPrimaryKey(value interface{}) string {
 	}
 
 	if indirectValue.IsValid() {
+		hasId := false
 		scopeTyp := indirectValue.Type()
 		for i := 0; i < scopeTyp.NumField(); i++ {
 			fieldStruct := scopeTyp.Field(i)
@@ -109,11 +110,16 @@ func GetPrimaryKey(value interface{}) string {
 			settings := parseTagSetting(fieldStruct.Tag.Get("gorm"))
 			if _, ok := settings["PRIMARY_KEY"]; ok {
 				return fieldStruct.Name
+			} else if fieldStruct.Name == "Id" {
+				hasId = true
 			}
 		}
+		if hasId {
+			return "Id"
+		}
 	}
 
-	return "Id"
+	return ""
 }
 
 func parseTagSetting(str string) map[string]string {