message: API: Refactor FixtureUtils getRandomTeacherForCourse
diff: import {User} from '../src/models/User';
  import {Course, ICourseModel} from '../src/models/Course';
  +import {IUserModel} from '../src/models/User';
  import {Lecture} from '../src/models/Lecture';
  import {Unit} from '../src/models/units/Unit';
  import {ICourse} from '../../shared/models/ICourse';
  @@ -39,12 +40,9 @@ export class FixtureUtils {
  return this.getRandom<IUser>(array, hash);
  }
  - public static async getRandomTeacherForCourse(course: ICourse, hash?: string): Promise<IUser> {
  - let array: IUser[] = [];
  - array = array.concat(course.teachers);
  - array.push(course.courseAdmin);
  - const user = await this.getRandom<IUser>(array, hash);
  - return User.findById(user);
  + public static async getRandomTeacherForCourse(course: ICourse, hash?: string): Promise<IUserModel> {
  + const user = await this.getRandom<IUser>([course.courseAdmin, ...course.teachers], hash);
  + return await User.findById(user);
  }
  public static async getRandomTeachers(min: number, max: number, hash?: string): Promise<IUser[]> {
  