Perform union in mongoDB

There are a couple of approaches to this that you can use the aggregate method for

db.collection.aggregate([    // Assign an array of constants to each document    { "$project": {        "linkedIn": 1,        "twitter": 1,        "source": { "$cond": [1, ["linkedIn", "twitter"],0 ] }    }},    // Unwind the array    { "$unwind": "$source" },    // Conditionally push the fields based on the matching constant    { "$group": {         "_id": "$_id",        "data": { "$push": {            "$cond": [                { "$eq": [ "$source", "linkedIn" ] },                { "source": "$source", "people": "$linkedIn.people" },                { "source": "$source", "people": "$twitter.people" }            ]        }}    }},    // Unwind that array    { "$unwind": "$data" },    // Unwind the underlying people array    { "$unwind": "$data.people" },    // Project the required fields    { "$project": {        "_id": 0,        "name": "$data.people.name",        "source": "$data.source"    }}])

Or with a different approach using some operators from MongoDB 2.6:

db.people.aggregate([    // Unwind the "linkedIn" people    { "$unwind": "$linkedIn.people" },    // Tag their source and re-group the array    { "$group": {        "_id": "$_id",        "linkedIn": { "$push": {            "name": "$linkedIn.people.name",            "source": { "$literal": "linkedIn" }        }},        "twitter": { "$first": "$twitter" }    }},    // Unwind the "twitter" people    { "$unwind": "$twitter.people" },    // Tag their source and re-group the array    { "$group": {        "_id": "$_id",        "linkedIn": { "$first": "$linkedIn" },        "twitter": { "$push": {            "name":  "$twitter.people.name",            "source": { "$literal": "twitter" }        }}    }},    // Merge the sets with "$setUnion"    { "$project": {        "data": { "$setUnion": [ "$twitter", "$linkedIn" ] }    }},    // Unwind the union array    { "$unwind": "$data" },    // Project the fields    { "$project": {        "_id": 0,        "name": "$data.name",        "source": "$data.source"    }}])

And of course if you simply did not care what the source was:

db.collection.aggregate([    // Union the two arrays    { "$project": {        "data": { "$setUnion": [            "$linkedIn.people",            "$twitter.people"        ]}    }},    // Unwind the union array    { "$unwind": "$data" },    // Project the fields    { "$project": {        "_id": 0,        "name": "$data.name",    }}])

Not sure if using aggregate is recommended over a map-reduce for that kind of operation but the following is doing what you're asking for (dunno if $const can be used with no issue at all in the .aggregate() function) :

aggregate([    { $project: { linkedIn: '$linkedIn', twitter: '$twitter', idx: { $const: [0,1] }}},   { $unwind: '$idx' },   { $group: { _id : '$_id', data: { $push: { $cond:[ {$eq:['$idx', 0]}, { source: {$const: 'LinkedIn'}, people: '$linkedIn.people' } , { source: {$const: 'Twitter'}, people: '$twitter.people' } ] }}}},   { $unwind: '$data'},   { $unwind: '$data.people'},   { $project: { _id: 0, name: '$data.people.name', source: '$data.source' }}])