在数组 mongodb 中使用 concat 时出现问题

Question

我有3张桌子

1) Actor: actor_id, first_name, last_name 
2) Film: film_id, title
3) Film_Actor: film_id, actor_id

示例文档：

_id
:
60aedac769985522a024daca
actor_id
:
"1"
first_name
:
"Penelope"
last_name
:
"Guiness"

我不仅要结果 first_name，还要 last_name。我在 $group 函数中使用 concat 时遇到问题。我的完整代码：

    db.film.aggregate([
  {
    $lookup: {
      from: "film_actor",
      localField: "film_id",
      foreignField: "film_id",
      as: "film_actor"
    }
  },
  {
    $lookup: {
      from: "actor",
      localField: "film_actor.actor_id", 
      foreignField: "actor_id",
      as: "actor"
    }
  },
  {
    $group: {
      _id: "$film_id",
      title: {"$first":"$title"},
      name: {$push: "$actor.first_name"}
                }
  }
]);

错误报告：

$concat only supports strings, not array

期望输出：

id:"207"
title:"Dangerous Uptown"
name:Array
0:"Penelope Guiness"
1:"Mary Watson"
2:"Ralph Holts"
3:"Spencer Dani"

Answer 1

感谢您所做的一切，我对您的代码做了一些修改

$lookup 加入 collections。我从 Flim collection
$unwind解构数组
$group重建我们已经解构的数组，这将
由于我们有嵌套数组，我们需要使用 $map 遍历它们以收集名字和姓氏
以上阶段将再次以嵌套数组结束，因此我们使用 $reduce 再次循环并使用 $setUnion

删除一些重复条目，具体取决于您的要求

这是代码

db.Film.aggregate([
  {
    $lookup: {
      from: "Film_Actor",
      localField: "film_id",
      foreignField: "film_id",
      as: "join_flim"
    }
  },
  { "$unwind": "$join_flim" },
  {
    $lookup: {
      from: "Actor",
      localField: "join_flim.actor_id",
      foreignField: "actor_id",
      as: "join_flim.join_actor"
    }
  },
  {
    $group: {
      _id: "$_id",
      title: { $first: "$title" },
      join_flim: { $push: "$join_flim" }
    }
  },
  {
    "$project": {
      title: 1,
      actornames: {
        $map: {
          input: "$join_flim",
          as: "f",
          in: {
            $map: {
              input: "$$f.join_actor",
              as: "a",
              in: {
                $concat: [ "$$a.first_name", " ", "$$a.last_name" ]
              }
            }
          }
        }
      }
    }
  },
  {
    "$project": {
      title: 1,
      actornames: {
        "$reduce": {
          "input": "$actornames",
          "initialValue": [],
          "in": {
            "$setUnion": [ "$$this", "$$value" ]
          }
        }
      }
    }
  }
])

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