Update/remove subdocument in MongoDB collection

Question

Note:Please use mongodb shell to execute the codes. Let's say i have one student document as below ``` { "_id" : 4, "grades" : [ { "grade" : 80, "mean" : 75, "std" : 8 }, { "grade" : 85, "mean" : 90, "std" : 5 }, { "grade" : 85, "mean" : 90, "std" : 5 }, { "grade" : 85, "mean" : 95, "std" : 6 }, { "grade" : 90, "mean" : 85, "std" : 5 } ] } ``` We have 2 problems : Problem 1 : lets say you wants to updates all subdocuments with _id=4 && grades.grade = 85 && grades.std = 5, with std=6 you will write as follows ``` db.students.update( {'$and':[ { _id: 4},{ "grades.grade": 85 }, {"grades.std": 5 } ]}, { $set: { "grades.$.std" : 6 } } ); ``` Now if you execute above statement multiple times(3 times) then ideally it should update 2nd,3rd sub-documents But last sub-document is also getting updated beacause it has std=5 match, but we have given condition as $and not $or, then how come last document is getting updated? Problem 2 : Now let's say you wants to remove the subdocument itself matchi…

Accepted Answer

The '$' operator only updates the first match so given:

```
db.students.insert({ "_id" : 4, "grades" : [ 
    { grade: 80, mean: 75, std: 8 }, 
    { grade: 85, mean: 90, std: 5 }, 
    { grade: 85, mean: 90, std: 5 }, 
    { grade: 85, mean: 95, std: 6 }, 
    { grade: 90, mean: 85, std: 5 } ]});
```

To update you need to target with `$elemMatch` like so:

```
db.students.update( { _id: 4, "grades": {$elemMatch: {"grade": 85, "std": 5 }}}, 
                    { $set: { "grades.$.std" : 6 } });
```

However, you have two grades that match `{"grades.grade": 85, "grades.std": 5}` and `$` only updates the first so you need to loop until all updated:

```
db.students.update( { _id: 4, "grades": {$elemMatch: {"grade": 85, "std": 5 }}}, 
                    { $set: { "grades.$.std" : 6 } })
while (db.getLastErrorObj()['n'] > 0) {
    db.students.update( { _id: 4, "grades": {$elemMatch: {"grade": 85, "std": 5 }}}, 
                        { $set: { "grades.$.std" : 6 } })
}
```

Problem 2: Same thing applies - you need to loop to `$pull` matching elements:

```
db.students.update({_id:4,'grades': {$elemMatch: {'grade':85}}}, {'$pull': {'grades': {'grade':85}}})
```

Update/remove subdocument in MongoDB collection

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