diff --git a/lib/ControlModel.ts b/lib/ControlModel.ts
index 165bf6b..9ced8b9 100644
--- a/lib/ControlModel.ts
+++ b/lib/ControlModel.ts
@@ -10,6 +10,7 @@ import { Persistent } from './Persistent';
 export abstract class ControlModel extends Persistent {
   public constructor (obj?: Record<string, string|number|boolean>) {
     super ();
+    this.define_properties ();
     for (const prop of Object.keys (this.properties)) {
       if ([
         'string',
@@ -43,4 +44,5 @@ export abstract class ControlModel extends Persistent {
   }
 
   public abstract verify(): void;
+  protected abstract define_properties(): void;
 }
diff --git a/lib/DatabaseModel.ts b/lib/DatabaseModel.ts
index 97468a8..dc7b0cb 100644
--- a/lib/DatabaseModel.ts
+++ b/lib/DatabaseModel.ts
@@ -15,6 +15,7 @@ export abstract class DatabaseModel extends Persistent {
   public constructor (id?: number) {
     super ();
     this.properties.id = 'number';
+    this.define_properties ();
     for (const prop of Object.keys (this.properties)) {
       if ([
         'string',
@@ -45,4 +46,5 @@ export abstract class DatabaseModel extends Persistent {
   public abstract async read(): Promise<boolean>;
   public abstract async write(): Promise<boolean>;
   public abstract async delete(): Promise<boolean>;
+  protected abstract define_properties(): void;
 }
diff --git a/lib/Persistent.ts b/lib/Persistent.ts
index 851d452..697d121 100644
--- a/lib/Persistent.ts
+++ b/lib/Persistent.ts
@@ -2,13 +2,7 @@ import { Assignable, Serializable } from './interfaces';
 
 export abstract class Persistent implements Assignable, Serializable {
   private _data: Record<string, unknown> = {};
-  protected readonly properties: Record<string, string>;
-
-  public constructor () {
-    this.properties = this.define_properties ();
-  }
-
-  protected abstract define_properties(): Record<string, string>;
+  protected readonly properties: Record<string, string> = {};
 
   public assign (a: Assignable): void {
     this.assign_object (a.to_object ());