在“pymongo (Python 3.7)”和“mongoDB”中使用 $cond 创建具有条件的新键值的问题

Question

I have a collection with the following document format:

{ 
   { "_id": 1234,
    "processes": [
         "0": { "0_0": aaaa , "0_1": bbbb },
         "1": { "1_0": cccc, "1_1": dddd },
         "2": { "2_0": eeee, "2_1": ffff},
     ]},
   { "_id": 5678,
    "processes": [
         "0": { "0_0": gggg, "0_1": hhhh},
         "1": { "1_0": iiii, "1_1": jjjj},
         "2": { "2_0": kkkk, "2_1": mmmm},
     ]}
}

In another query I made about the same DB, my colleague @hhharsha36 helped me with the problem I had:

Update with Pymongo boolean field in a subdocument within a list field of a document in a MongoDB collection

Thanks to that my current aggregate query looks like this:

cursor_processes = collection.aggregate([
  {
    "$unwind": "$processes"
  },
  {
    "$replaceRoot": {
      "newRoot": {
        "$mergeObjects": [
          {
            "_id": "$_id"
          },
          "$processes"
        ]
      }
    }
  }
])

At this moment, I want to create a key-value that depends on whether or not a key named motive exists previously. Then:

cursor_processes = collection.aggregate([
  {
    "$unwind": "$processes"
  },
  {
    "$replaceRoot": {
      "newRoot": {
        "$mergeObjects": [
          {
            "_id": "$_id"
          },
          "$processes",
      {
        "code": {
               '$cond': [
                  {"processes.0": {'$exists':  True}},
                  {'$concat': ["$processes.0_0", "$processes.0_1", {'$substr': ["$_id", 0, -1]}, "si_00"]}
                  {'$concat': ["$processes.1_0", "$processes.2_0", {'$substr': ["$_id", 0, -1]}, "no_00"]}
                ]
            }
      }
        ]
      }
    }
  }
])

list_proc = [i for i in cursor_processes] #Create a list

When I debug, I get the following error message:

pymongo.errors.OperationFailure: Unrecognized expression '$exists', full error: {'ok': 0.0, 'errmsg': "Unrecognized expression '$exists'", 'code': 168, 'codeName': 'InvalidPipelineOperator'}

Please help me. Thanks.

Answer 1

$exists is a query operator.

$cond is expecting that first argument to be a boolean expression, not a filter document.

You might try the $ifNull pipeline operator, or perhaps a combination of $eq and $not pipeline operators