MapReduce function MongoDB NodeJs

Question

I am trying to return a field of every object in my collection in form of array using mapReduce function. These are my documents in a collection.

 { _id: '1', name: 'a' },
 { _id: '2', name: 'b' },
 { _id: '4', name: 'c' },
 { _id: '5', name: 'd' },
 { _id: '6', name: 'e' },
 { _id: '7', name: 'f' }

Now i want result in this form ['a','b','c','d','e','f']. How i can achieve it, i tried mapReduce but couldn't get the result in this way.

This is my code

collection.mapReduce( function EachBranch(  ) {
      emit( this.name, this.value);
      }, function ( key, values ) {
      },{ out: { inline: 1 } });

Saleem · Accepted Answer · 2016-10-22 00:45:55Z

2

You'll need to iterate over values in reducer and transform result in desired form.

Example: Try in mongo shell

db.collection.mapReduce(
  function() {
    emit(1, this.name)
  },
  function(k,v){
    var result = {};
    result.names = v;
    return result;
  },
  {out: {inline:1}}
).results[0].value.names;

Based on your sample input documents, you'll get output as:

[ "a", "b", "c", "d", "e", "f" ]

Update: Node.js solution:

collection.mapReduce(
    function () {
        emit(1, this.name)
    },
    function (k, v) {
        var result = {};
        result.names = v;
        return result;
    },
    { out: { inline: 1 } },
    function (err, result) {
        assert.equal(null, err);

        if (result) {
            console.log(result[0].value.names);
        }
        db.close();
    }
);

Note: I'm not handling any error so please do defensive coding.

edited Oct 22, 2016 at 0:45

answered Oct 21, 2016 at 13:12

Saleem

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5 Comments

Saleem Over a year ago

See my NOTE above. I clearly mentioned that I'm not doing any error check. If there is no result, you'll get error. I am leaving error handling on you. Please note, SO is not a coding service. We are here to help but not to solve assignments or do job.

Abdul Bari Over a year ago

I understand, the issue is in your code the results is undefined

Saleem Over a year ago

Have you had a try? Copy above code and run in shell.

Abdul Bari Over a year ago

in my understanding .results.... is part of mongo and can't be used in nodejs? So I have to map the returned values and take out name from it?

Dirigible Over a year ago

What is emit()?

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