[mongodb] mongodb: how to compare String from document to the String constructed by expression in aggregation pipeline

[Questioin]

I have a trouble with creating a view in MongoDB. I'm just starting with Mongo, so the question could look pretty silly.

I have the following document:

{ 
    "_id" : NumberInt(12213054), 
    ... 
    "_dt" : {
        "_doc" : "time", 
        "time" : "01:30", 
        "date" : "04/02/18", 
        "tz" : "CET", 
        "tzoffset" : NumberInt(3600), 
        "uts" : NumberInt(1517704200)
    }, 
    ...
}

I'm trying to create a view for documents that have a date '04/02/18'. The type of "_dt.date" field is String. And I'm trying to compare it using $eq operator.

I have the following query to select the data:

{$match: {
    "_dt.date": {
        $eq: { 
            $concat: [  
                {$dateToString: {
                    format: "%d/%m/",
                    date: new Date("2018-02-04")
                }},
                {$substr: [
                    {$year : new Date("2018-02-04")},
                    2,
                    2
                ]}
            ]
        }
    }
}
}

But after I execute the query the result is empty.

The date string constructed by the subquery seems to be correct:

db.matches.aggregate({ $project: { date:
{ 
            $concat: [
                {$dateToString: {
                    format: "%d/%m/",
                    date: new Date("2018-02-04")
                }},
                {$substr: [
                    {$year : new Date("2018-02-08")},
                    2,
                    2
                ]}
            ]
        }
}
}
)

Output is:

{ 
    "_id" : NumberInt(12213054), 
    "date" : "04/02/18"
}

Does anyone have any idea why the first query doesn't work?

[Answer]

$match by design compares a field against a constant value with regular query operators.

For comparing two fields in mongodb you've to use $match with $expr with aggregation query operators in 3.6.

Compare query operators vs aggregation comparison operators.

{"$match":{
  "$expr":{
    "$eq":[
      "$_dt.date", 
      {"$concat":[
        {"$dateToString":{"format":"%d/%m/","date":new Date("2018-02-04")}},
        {"$substr":[{"$year":new Date("2018-02-04")},2,2]}
      ]}
  }
}}

Or

You can simply pass the date string created on client side and compare directly

{"$match":{"_dt.date":"04/02/18"}}

[출처] https://stackoverflow.com/questions/48727055/mongodb-how-to-compare-string-from-document-to-the-string-constructed-by-expres

[mongodb] mongodb: how to compare String from document to the String constructed by expression in aggregation pipeline

[Questioin]

I have a trouble with creating a view in MongoDB. I'm just starting with Mongo, so the question could look pretty silly.

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I have the following document:

{ 
    "_id" : NumberInt(12213054), 
    ... 
    "_dt" : {
        "_doc" : "time", 
        "time" : "01:30", 
        "date" : "04/02/18", 
        "tz" : "CET", 
        "tzoffset" : NumberInt(3600), 
        "uts" : NumberInt(1517704200)
    }, 
    ...
}

I'm trying to create a view for documents that have a date '04/02/18'. The type of "_dt.date" field is String. And I'm trying to compare it using $eq operator.

I have the following query to select the data:

{$match: {
    "_dt.date": {
        $eq: { 
            $concat: [  
                {$dateToString: {
                    format: "%d/%m/",
                    date: new Date("2018-02-04")
                }},
                {$substr: [
                    {$year : new Date("2018-02-04")},
                    2,
                    2
                ]}
            ]
        }
    }
}
}

But after I execute the query the result is empty.

The date string constructed by the subquery seems to be correct:

db.matches.aggregate({ $project: { date:
{ 
            $concat: [
                {$dateToString: {
                    format: "%d/%m/",
                    date: new Date("2018-02-04")
                }},
                {$substr: [
                    {$year : new Date("2018-02-08")},
                    2,
                    2
                ]}
            ]
        }
}
}
)

Output is:

{ 
    "_id" : NumberInt(12213054), 
    "date" : "04/02/18"
}

Does anyone have any idea why the first query doesn't work?

[Answer]

$match by design compares a field against a constant value with regular query operators.

For comparing two fields in mongodb you've to use $match with $expr with aggregation query operators in 3.6.

Compare query operators vs aggregation comparison operators.

{"$match":{
  "$expr":{
    "$eq":[
      "$_dt.date", 
      {"$concat":[
        {"$dateToString":{"format":"%d/%m/","date":new Date("2018-02-04")}},
        {"$substr":[{"$year":new Date("2018-02-04")},2,2]}
      ]}
  }
}}

Or

You can simply pass the date string created on client side and compare directly

{"$match":{"_dt.date":"04/02/18"}}

[출처] https://stackoverflow.com/questions/48727055/mongodb-how-to-compare-string-from-document-to-the-string-constructed-by-expres