code: function containmentOperator(queryObject, actualField, key) {
      let substitution = {};
  
      substitution[key === '$in' ? '$or' : '$and'] = queryObject.map(value => {
          let expression = {};
          expression[actualField] = {};
          expression[actualField][key === '$in' ? '$eq' : '$ne'] = value;
  
          return expression;
      });
  
      return substitution;
  }
docs: Transform containment operators ($in, $nin) to 'and', 'or' logical operations
  which could be handled easily by the parser. E.g: field: {$in: [1, 2]} -> field = 1 or field = 2
  
  @param {object} queryObject Which contains the operator.
  @param {string} actualField Current field of the object which the filter is operating on.
  @param {string} key Should be '$in' or '$nin'. The current operation key.
  @returns {object} Translated query