Add TreeModel and HasSubtree implementation

src/orm/model/relation/HasOneOrMany.ts

@@ -59,7 +59,7 @@ export abstract class HasOneOrMany<T extends Model<T>, T2 extends Model<T2>, V e
     public applyScope(where: AbstractBuilder<T2>): void {
         where.where(subq => {
             subq.where(this.qualifiedForeignKey, '=', this.parentValue)
-                .whereRaw(this.qualifiedForeignKey, 'IS NOT', 'NULL')
+                .whereNotNull(this.qualifiedForeignKey)
         })
     }