diff --git a/src/main/kotlin/fr/postgresjson/entity/Entity.kt b/src/main/kotlin/fr/postgresjson/entity/Entity.kt
index f770ece..83778d3 100644
--- a/src/main/kotlin/fr/postgresjson/entity/Entity.kt
+++ b/src/main/kotlin/fr/postgresjson/entity/Entity.kt
@@ -47,47 +47,45 @@ class EntityCreatedAtImp: EntityCreatedAt {
 class EntityUpdatedAtImp: EntityUpdatedAt {
     override var updatedAt: DateTime? = null
 }
-
 interface User<T>: EntityI<T> {
-    fun isValid(): Boolean
 }
 
 /* Author */
-interface CreatedBy<T> {
-    var createdBy: User<T>?
+interface CreatedBy<T: User<*>> {
+    var createdBy: T?
 }
 
-interface UpdatedBy<T> {
-    var updatedBy: User<T>?
+interface UpdatedBy<T: User<*>> {
+    var updatedBy: T?
 }
 
-class EntityCreatedByImp<T>: CreatedBy<T> {
-    override var createdBy: User<T>? = null
+class EntityCreatedByImp<UserT: User<*>>: CreatedBy<UserT> {
+    override var createdBy: UserT? = null
 }
 
-class EntityUpdatedByImp<T>: UpdatedBy<T> {
-    override var updatedBy: User<T>? = null
+class EntityUpdatedByImp<UserT: User<*>>: UpdatedBy<UserT> {
+    override var updatedBy: UserT? = null
 }
 
 /* Published */
-interface Published<UserT> {
+interface Published<UserT: User<*>> {
     var publishedAt: DateTime?
-    var publishedBy: User<UserT>?
+    var publishedBy: UserT?
 }
 
-class EntityPublishedImp<UserT>: Published<UserT> {
+class EntityPublishedImp<UserT: User<*>>: Published<UserT> {
     override var publishedAt: DateTime? = null
-    override var publishedBy: User<UserT>? = null
+    override var publishedBy: UserT? = null
 }
 
 /* Implementation */
-abstract class EntityImp<T, UserT>: Entity<T>(),
+abstract class EntityImp<T, UserT: User<*>>: Entity<T>(),
     EntityCreatedAt by EntityCreatedAtImp(),
     EntityUpdatedAt by EntityUpdatedAtImp(),
     CreatedBy<UserT> by EntityCreatedByImp(),
     UpdatedBy<UserT> by EntityUpdatedByImp()
 
-abstract class EntityExtended<T, UserT>:
+abstract class EntityExtended<T, UserT: User<*>>:
     EntityImp<T, UserT>(),
     EntityVersioningIncrement by EntityVersioningIncrementImp(),
     Published<UserT> by EntityPublishedImp()
diff --git a/src/test/kotlin/fr/postgresjson/EntityTest.kt b/src/test/kotlin/fr/postgresjson/EntityTest.kt
index 074a26e..8e0840f 100644
--- a/src/test/kotlin/fr/postgresjson/EntityTest.kt
+++ b/src/test/kotlin/fr/postgresjson/EntityTest.kt
@@ -7,18 +7,19 @@ import org.junit.jupiter.api.TestInstance
 
 @TestInstance(TestInstance.Lifecycle.PER_CLASS)
 class EntityTest() {
-    private class ObjTest(var name: String): EntityExtended<Int?, Int?>()
+    private class User(override var id: Int?): fr.postgresjson.entity.User<Int?>
+    private class ObjTest(var name: String): EntityExtended<Int?, User>()
 
     @Test
     fun getObject() {
         val obj: ObjTest? = ObjTest("plop")
         assertTrue(obj is ObjTest)
-        assertTrue(obj is EntityExtended<Int?, Int?>)
+        assertTrue(obj is EntityExtended<Int?, User>)
         assertTrue(obj is EntityI<Int?>)
         assertTrue(obj is Entity<Int?>)
-        assertTrue(obj is Published<Int?>)
-        assertTrue(obj is CreatedBy<Int?>)
-        assertTrue(obj is UpdatedBy<Int?>)
+        assertTrue(obj is Published<User>)
+        assertTrue(obj is CreatedBy<User>)
+        assertTrue(obj is UpdatedBy<User>)
         assertTrue(obj is EntityCreatedAt)
         assertTrue(obj is EntityUpdatedAt)
     }