MongoDB根据id对文档进行分组，并合并文档中匹配的数组元素

[
  { "_id": 1, "name": "Alice", "projects": ["ProjectA", "ProjectB"] },
  { "_id": 2, "name": "Bob", "projects": ["ProjectC"] },
  { "_id": 1, "name": "Alice", "projects": ["ProjectD"] }
]

db.users.aggregate([
  {
    $group: {
      _id: "$_id",
      name: { $first: "$name" },
      projects: { $push: "$projects" }
    }
  },
  {
    $project: {
      _id: 1,
      name: 1,
      projects: { $reduce: { input: "$projects", initialValue: [], in: { $concatArrays: ["$$value", "$$this"] } } }
    }
  }
])

[
  { "_id": 1, "name": "Alice", "projects": ["ProjectA", "ProjectB", "ProjectD"] },
  { "_id": 2, "name": "Bob", "projects": ["ProjectC"] }
]