要过滤并仅返回以A8结尾的路径，您必须在$graphLookup之后再次使用$match stage**。
您可以尝试类似这样的操作，仅返回以A8结尾的路径

db.collection.aggregate([
  {
    "$match": {
      sign1: "A1"
    }
  },
  {
    "$graphLookup": {
      "from": "collection",
      "startWith": "$sign1",
      "connectFromField": "sign2",
      "connectToField": "sign1",
      "as": "path",
      "depthField": "step"
    }
  },
  {
    "$match": {
      "path.sign2": "A8"
    }
  }
])

现在，如果您想将路径分隔到不同的数组/文档中，则必须在$match阶段之前使用$unwind stage，如下所示：

db.collection.aggregate([
  {
    "$match": {
      sign1: "A1"
    }
  },
  {
    "$graphLookup": {
      "from": "collection",
      "startWith": "$sign1",
      "connectFromField": "sign2",
      "connectToField": "sign1",
      "as": "path",
      "depthField": "step"
    }
  },
  {
    "$unwind": "$path"
  },
  {
    "$match": {
      "path.sign2": "A8"
    }
  }
])

EDITED：如果您想过滤掉不需要的路径或step等于0的路径，您可以在$graphLookup阶段使用$filter操作，如下所示，根据这两个条件返回结果。

db.collection.aggregate([
  {
    "$match": {
      sign1: "A1"
    }
  },
  {
    "$graphLookup": {
      "from": "collection",
      "startWith": "$sign1",
      "connectFromField": "sign2",
      "connectToField": "sign1",
      "as": "path",
      "depthField": "step",
      "maxDepth": 10,
      "filter": {
        "$and": [
          { "step": { "$gt": 0 } },
          { "sign2": "A8" }
        ]
      }
    }
  },
  {
    "$unwind": "$path"
  }
])

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