diff --git a/src/Entity/Field/Date.php b/src/Entity/Field/Date.php
index fe0df70..c2c5e7e 100644
--- a/src/Entity/Field/Date.php
+++ b/src/Entity/Field/Date.php
@@ -4,11 +4,8 @@ namespace Ulmus\Entity\Field;
 
 class Date extends Datetime {
     
-    public function __toString()
-    {
-        return $this->format("Y-m-d");
-    }
-
+    public string $format = "Y-m-d";
+    
     public function formatLocale(string $format) : string
     {
         return strftime($format, $this->getTimestamp());
diff --git a/src/Entity/Field/Time.php b/src/Entity/Field/Time.php
index ecbdd11..df4db07 100644
--- a/src/Entity/Field/Time.php
+++ b/src/Entity/Field/Time.php
@@ -4,9 +4,6 @@ namespace Ulmus\Entity\Field;
 
 class Time extends Datetime {
     
-    public function __toString()
-    {
-        return $this->format("H:i:s");
-    }
+    public string $format = "H:i:s";
     
 }
diff --git a/src/Query/GroupBy.php b/src/Query/GroupBy.php
index 7a05b4b..fd969cf 100644
--- a/src/Query/GroupBy.php
+++ b/src/Query/GroupBy.php
@@ -3,22 +3,23 @@
 namespace Ulmus\Query;
 
 class GroupBy extends Fragment {
-    
-    const SQL_TOKEN = "GROUP BY";
-    
-    public int $order = 70;
+    public int $order = 75;
 
     public array $groupBy = [];
+    
+    const SQL_TOKEN = "GROUP BY";
 
     public function set(array $order) : self
     {
-        $this->groupBy = $order;
+        $this->groupBy = [ $order ];
+        
         return $this;
     }
 
-    public function add(string $field, ? string $direction = null) : self
+    public function add(string $field) : self
     {
         $this->groupBy[] = $field;
+        
         return $this;
     }
 
diff --git a/src/Query/Having.php b/src/Query/Having.php
index 5d349cd..d168d0b 100644
--- a/src/Query/Having.php
+++ b/src/Query/Having.php
@@ -4,5 +4,6 @@ namespace Ulmus\Query;
 
 class Having extends Where {
     const SQL_TOKEN = "HAVING";
-    public int $order = 75;
+    
+    public int $order = 80;
 }
diff --git a/src/Query/OrderBy.php b/src/Query/OrderBy.php
index 5e62ad2..318690a 100644
--- a/src/Query/OrderBy.php
+++ b/src/Query/OrderBy.php
@@ -3,7 +3,7 @@
 namespace Ulmus\Query;
 
 class OrderBy extends Fragment {
-    public int $order = 80;
+    public int $order = 85;
 
     public array $orderBy = [];
     
diff --git a/src/QueryBuilder.php b/src/QueryBuilder.php
index c597c79..80301a2 100644
--- a/src/QueryBuilder.php
+++ b/src/QueryBuilder.php
@@ -216,11 +216,6 @@ class QueryBuilder
         return $this->where($field, $value, $operator, true);
     }
 
-    public function groupBy() : self
-    {
-        return $this;
-    }
-
     public function limit(int $value) : self
     {
         if ( null === $limit = $this->getFragment(Query\Limit::class) ) {
@@ -257,6 +252,18 @@ class QueryBuilder
         return $this;
     }
 
+    public function groupBy(string $field, ? string $direction = null) : self
+    {
+        if ( null === $groupBy = $this->getFragment(Query\GroupBy::class) ) {
+            $groupBy = new Query\GroupBy();
+            $this->push($groupBy);
+        }
+
+        $groupBy->add($field, $direction);
+        
+        return $this;
+    }
+
     public function join(string $type, /*string | QueryBuilder*/ $table, $field, $value, bool $outer = false, ? string $alias = null) : self
     {
         $join = new Query\Join($this);