MongoDB - Aggregation - To get unique items in array

Question

Here\'s my MongoDB collection:

{
    \"_id\" : ObjectId(\"515d8f53175b8ecb053425c2\"),
    \"category\" : \"Batteries\",
    \"products\" : [
        {
              


        

Answer 1

After mongodb3.4, there is a $reduce operator, so we can flat a array without extra stage.

1.

col.aggregate([
  {
    $project: {
      items: {
        $reduce: {
          input: "$products.items",
          initialValue: [],
          in: { $concatArrays: ["$$value", "$$this"] },
        },
      },
    },
  },
  { $unwind: "$items" },
  { $group: { _id: null, items: { $addToSet: "$items" } } },
]);

2.

col.aggregate([
  {
    $project: {
      category: 1,
      items: {
        $setUnion: {
          $reduce: {
            input: "$products.items",
            initialValue: [],
            in: { $concatArrays: ["$$value", "$$this"] },
          },
        },
      },
    },
  },
]);