Implemented new Permission system on the DB side.

ref #22

src/models/entities/UserGroup.ts

@@ -1,29 +1,16 @@
 import {
-  IsInt,
   IsNotEmpty,
   IsOptional,
   IsString
 } from "class-validator";
-import { Column, Entity, ManyToOne, PrimaryGeneratedColumn } from "typeorm";
-import { Permission } from "./Permission";
+import { ChildEntity, Column } from "typeorm";
+import { Principal } from './Principal';
 
 /**
  * Defines the UserGroup interface.
 */
-@Entity()
-export class UserGroup {
-  /**
-   * Autogenerated unique id (primary key).
-   */
-  @PrimaryGeneratedColumn()
-  @IsInt()
-  id: number;
-
-  /**
-     * permissions
-     */
-  @ManyToOne(() => Permission, permission => permission.groups, { nullable: true })
-  permissions: Permission[];
+@ChildEntity()
+export class UserGroup extends Principal {
 
   /**
    * The group's name