mongodb aggregation add field derived from another field

Question

here is my data which I need to add in a collection

let data = {
    'a':'data a',
    'ab':'data a',
    'b':'data b',
    'c':'data c'
}

here is my collection data:

{
    name:'xyz',
    age:'100',
    arr: ['a','ab']
}

what I want:

{
    name:'xyz',
    age:'100',
    arr: ['a','ab'],
    data:'data a' //get this from arr first element, and that is key in data object that's value here 
}

can someone help me to write this query in mongoose?

Answer 1

You need an aggregate operation that makes use of the following pipeline stages and operators:

• $addFields: adds a new field to the pipeline documents
• $objectToArray: converts an object/document to an array. This is necessary for checking the index of matching array elements
• $arrayElemAt: gets the element at the specified array index
• $indexOfArray: gets the the array index (zero-based) of the first occurrence of a specified value

Your final pipeline should be as follows:

let data = {
    'a':'data a',
    'ab':'data a',
    'b':'data b',
    'c':'data c'
};

db.collection.aggregate([
    { $addFields: {
        data: {  $objectToArray:  data  },
    } },
    { $addFields: {
        data: {
            $arrayElemAt:  [
                '$data.v',
                {
                    $indexOfArray: [ 
                        '$data.k', 
                        {  $first: '$arr' } 
                    ] 
                }  
            ]
        }
    } }
])

OR

db.collection.aggregate([
    { $addFields: {
        data: {  $objectToArray:  data  },
    } },
    { $addFields: {
        data: {
            $arrayElemAt:  [
                '$data.v',
                {
                    $indexOfArray: [ 
                        '$data.k', 
                        {  $arrayElemAt: ['$arr', 0] } 
                    ] 
                }  
            ]
        }
    } }
])

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