{
  "_comment": [
    "Inputs only. Expectations are measured -- see record.js.",
    "`$`, the original positional operator, which writes the *first* element",
    "the query matched. It is the one of the three that needs something the",
    "document alone does not hold -- which element the filter matched -- so",
    "every case here pairs a filter with an update and the pairing is the",
    "point. The pinned crud corpus has no `$` case anywhere."
  ],
  "documents": [
    { "_id": 1, "y": [{ "b": 3 }, { "b": 1 }] },
    { "_id": 2, "y": [{ "b": 0 }, { "b": 1 }] },
    { "_id": 3, "y": [{ "b": 5, "c": [{ "d": 2 }] }] }
  ],
  "cases": [
    {
      "description": "$ writes the element the filter matched",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 3 },
        "update": { "$set": { "y.$.b": 7 } }
      }
    },
    {
      "description": "$ writes only the first match when several elements qualify",
      "documents": [{ "_id": 1, "y": [{ "b": 1 }, { "b": 1 }, { "b": 2 }] }],
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 1 },
        "update": { "$set": { "y.$.b": 7 } }
      }
    },
    {
      "description": "$ when the filter never touched the array is refused",
      "operation": "updateOne",
      "arguments": {
        "filter": { "_id": 1 },
        "update": { "$set": { "y.$.b": 7 } }
      }
    },
    {
      "description": "$inc through $",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 3 },
        "update": { "$inc": { "y.$.b": 10 } }
      }
    },
    {
      "description": "$ as the leaf replaces the matched element",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 3 },
        "update": { "$set": { "y.$": { "replaced": true } } }
      }
    },
    {
      "description": "$ reaching a field below the matched element",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 5 },
        "update": { "$set": { "y.$.c": [] } }
      }
    },
    {
      "description": "$ with the filter matching on a different field of the element",
      "documents": [{ "_id": 1, "y": [{ "k": "a", "v": 1 }, { "k": "b", "v": 2 }] }],
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.k": "b" },
        "update": { "$set": { "y.$.v": 9 } }
      }
    },
    {
      "description": "updateMany with $ writes each document's own match",
      "operation": "updateMany",
      "arguments": {
        "filter": { "y.b": 1 },
        "update": { "$set": { "y.$.b": 7 } }
      }
    },
    {
      "description": "$ over an array of scalars",
      "documents": [{ "_id": 1, "y": [1, 2, 3] }],
      "operation": "updateOne",
      "arguments": {
        "filter": { "y": 2 },
        "update": { "$set": { "y.$": 9 } }
      }
    },
    {
      "description": "$ in first position is refused",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 3 },
        "update": { "$set": { "$": 7 } }
      }
    },
    {
      "description": "$ twice in one path is refused",
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 5 },
        "update": { "$set": { "y.$.c.$.d": 0 } }
      }
    },
    {
      "description": "$ on an upsert that inserts",
      "documents": [],
      "operation": "updateOne",
      "arguments": {
        "filter": { "y.b": 3 },
        "update": { "$set": { "y.$.b": 7 } },
        "upsert": true
      }
    }
  ]
}