Mongodb中Group by数组元素

[
  { "_id": 1, "customer": "John", "items": ["apple", "banana", "orange"] },
  { "_id": 2, "customer": "John", "items": ["apple", "banana"] },
  { "_id": 3, "customer": "Jane", "items": ["apple", "orange"] },
  { "_id": 4, "customer": "John", "items": ["banana", "orange"] }
]

db.orders.aggregate([
  { $unwind: "$items" },
  { $group: {
    _id: { customer: "$customer", item: "$items" },
    count: { $sum: 1 }
  }},
  { $group: {
    _id: "$_id.customer",
    items: { $push: { item: "$_id.item", count: "$count" } },
    totalCount: { $sum: "$count" }
  }}
])

[
  {
    "_id": "John",
    "items": [
      { "item": "apple", "count": 2 },
      { "item": "banana", "count": 2 },
      { "item": "orange", "count": 2 }
    ],
    "totalCount": 6
  },
  {
    "_id": "Jane",
    "items": [
      { "item": "apple", "count": 1 },
      { "item": "orange", "count": 1 }
    ],
    "totalCount": 2
  }
]