下面是另一种方法，使用"$reduce"。注解在聚合管道中。

db.collection.aggregate([
  {
    "$set": {
      // rewrite fruits
      "fruits": {
        "$reduce": {
          "input": "$fruits",
          "initialValue": [],
          "in": {
            "$let": {
              "vars": {
                // get fruit index in $$value : will be -1 if not there
                "idx": {"$indexOfArray": ["$$value.fruit", "$$this.fruit"]}
              },
              "in": {
                "$cond": [
                  // is fruit not in $$value yet
                  {"$eq": ["$$idx", -1]},
                  // new fruit so put in $$value and make "type" an array
                  {
                    "$concatArrays": [
                      "$$value",
                      [{"$mergeObjects": ["$$this", {"type": ["$$this.type"]}]}]
                    ]
                  },
                  // fruit already in $$value, so map $$value with "type" update
                  {
                    "$map": {
                      "input": "$$value",
                      "as": "val",
                      "in": {
                        "$cond": [
                          // is this array element not the right fruit?
                          {"$ne": ["$$val.fruit", "$$this.fruit"]},
                          // nope, leave the element as-is
                          "$$val",
                          // this element needs to be updated
                          {
                            "$mergeObjects": [
                              "$$val",
                              {
                                "type": {
                                  "$cond": [
                                    // is this "type" already in array?
                                    {"$in": ["$$this.type", "$$val.type"]},
                                    // yes, so leave it as-is
                                    "$$val.type",
                                    // this is a new "type", so add it to array
                                    {"$concatArrays": ["$$val.type", ["$$this.type"]]}
                                  ]
                                }
                              }
                            ]
                          }
                        ]
                      }
                    }
                  }
                ]
              }
            }
          }
        }
      }
    }
  }
])

在mongoplayground.net上试试。

也许是这样的：

db.collection.aggregate([
{
  $unwind: "$fruits"
},
{
 $group: {
  _id: "$fruits.fruit",
  type: {
    $push: "$fruits.type"
  }
 }
},
{
 $project: {
  fruit: "$_id",
  type: 1,
  _id: 0
 }
},
{
$group: {
  _id: "",
  fruits: {
    $push: "$$ROOT"
  }
 }
 }
])

解释：
1.展开阵列
1.组以形成类型数组（如果只需要unique，可以使用$push或$addToSet）
1.投影必要的字段
1.将所有文档分组到单个最终文档中
Playground

这是另一种方法，使用多个"$map"和"$setUnion"来获得唯一的数组成员。

db.collection.aggregate([
  {
    "$set": {
      // rewrite fruits
      "fruits": {
        "$map": {
          // map over unique fruits
          "input": {"$setUnion": "$fruits.fruit"},
          "as": "theFruit",
          "in": {
            // set fruit
            "fruit": "$$theFruit",
            // "type" are unique elements of fruits.type
            // where fruits.fruit == theFruit
            "type": {
              "$setUnion": {
                "$map": {
                  "input": {
                    "$filter": {
                      "input": "$fruits",
                      "as": "obj",
                      "cond": {"$eq": ["$$obj.fruit", "$$theFruit"]}
                    }
                  },
                  "in": "$$this.type"
                }
              }
            }
          }
        }
      }
    }
  }
])

在mongoplayground.net上试试。