Surveys.aggregate.js

return Surveys.aggregate(
  { $match: query },
  { $group: {
    _id: dateGroup,
    participants: { $sum: '$ratersCount' },
    rating: { $avg: '$averageRating' },
    surveys: { $push: '$_id' },
    selectedTags: { $push: '$tags' },
    peoples: { $addToSet: '$fullname' },
  } },
  { $project: {
    _id: null,
    selectedTags: {
      $reduce: {
        input: "$selectedTags",
        initialValue: [],
        in: { $setUnion: ["$$value", "$$this"] }
      }
    },
  } }
);