text: import _ from 'lodash';
  import { isMongoId } from 'validator';
  
  class MongoIds {
    constructor() {
      this.knownIds = [];
    }
    check(id, title) {
      if (!isMongoId(id)) {
        throw new Error(`Expected a valid ObjectId for ${title}, but got ${id}`);
      }
      const idIndex = _.findIndex(this.knownIds, existing => id === existing);
      if (idIndex !== -1) {
        throw new Error(`
      All challenges must have a unique id.
  
      The id for ${title} is already assigned
      `);
      }
      this.knownIds = [ ...this.knownIds, id ];
    }
  }
  
  export default MongoIds;
  