remove specific subdocument mongo in array

Question

I have the following document :

    {
    "grades" : [
        {
            "grade" : 80,
            "mean" : 75,
            "std" : 8,
            "otro" : [
                {
                    "i" : 1
                },
                {
                    "i" : 2
                }
            ]
        },
        {
            "grade" : 85,
            "mean" : 90,
            "std" : 5
        },
        {
            "grade" : 85,
            "mean" : 90,
            "std" : 5,
            "otro" : [
                {
                    "i" : 3
                },
                {
                    "i" : 4
                }
            ]
        },
        {
            "grade" : 85,
            "mean" : 95,
            "std" : 6
        },
        {
            "grade" : 90,
            "mean" : 85,
            "std" : 5,
            "otro" : [
                {
                    "i" : 5
                },
                {
                    "i" : 6
                }
            ]
        }
    ]
}

What i want to do it's to remove the sub document where 'otro.i' : 2, i have tried this :

db.ejemplo.update(
    {
        _id: oid,
        'grades':
        {
            $elemMatch:
            {
                'otro':
                {
                    $elemMatch:
                    {
                        'i': 2
                    }
                }
            }
        }
    },
    {
        '$pull':
        {
            'grades':
            {
                'otro':
                {
                    'i':2
                }
            }
        }
    }
);

But this removes the all subdocument 'otro' where the key 'i' is equal to 2 ,

this is before :

{
            "grade" : 80,
            "mean" : 75,
            "std" : 8,
            "otro" : [
                {
                    "i" : 1
                },
                {
                    "i" : 2
                }
            ]
        }

this is after

{
            "grade" : 80,
            "mean" : 75,
            "std" : 8
        }

But the result i want is :

{
            "grade" : 80,
            "mean" : 75,
            "std" : 8,
            "otro" : [
                {
                    "i" : 1
                }
            ]
        }

Any ideas ?

Answer 1

Use the $ positional operator to update the document. This identifies an element in an array to update without explicitly specifying the position of the element in the array:

db.ejemplo.update(
{
    _id: oid,
    'grades':
        {
            $elemMatch:
            {
                'otro':
                {
                    $elemMatch:
                    {
                        'i': 2
                    }
                }
            }
        }
    },
    {
        '$pull': {
            'grades.$.otro': {"i": 2}                
        }
    }
)