How do I rename fields when performing search/projection in MongoDB?

So basically using .aggregate() instead of .find():

db.tweets.aggregate([
    { "$project": {
        "_id": 0,
        "coords": "$level1.level2.coordinates"
    }}
])

And that gives you the result that you want.

MongoDB 2.6 and above versions return a "cursor" just like find does.

See $project and other aggregation framework operators for more details.

For most cases you should simply rename the fields as returned from .find() when processing the cursor. For JavaScript as an example, you can use .map() to do this.

From the shell:

db.tweets.find({},{'level1.level2.coordinates': 1, _id:0}).map( doc => {
  doc.coords = doc['level1']['level2'].coordinates;
  delete doc['level1'];
  return doc;
})

Or more inline:

db.tweets.find({},{'level1.level2.coordinates': 1, _id:0}).map( doc => 
  ({ coords: doc['level1']['level2'].coordinates })
)

This avoids any additional overhead on the server and should be used in such cases where the additional processing overhead would outweigh the gain of actual reduction in size of the data retrieved. In this case ( and most ) it would be minimal and therefore better to re-process the cursor result to restructure.

As mentioned by @Neil Lunn this can be achieved with an aggregation pipeline:

And starting Mongo 4.2, the $replaceWith aggregation operator can be used to replace a document by a sub-document:

// { level1: { level2: { coordinates: [10, 20] }, b: 4 }, a: 3 }
db.collection.aggregate(
  { $replaceWith: { coords: "$level1.level2.coordinates" } }
)
// { "coords" : [ 10, 20 ] }

Since you mention findOne, you can also limit the number of resulting documents to 1 as such:

db.collection.aggregate([
  { $replaceWith: { coords: "$level1.level2.coordinates" } },
  { $limit: 1 }
])

Prior to Mongo 4.2 and starting Mongo 3.4, $replaceRoot can be used in place of $replaceWith:

db.collection.aggregate(
  { $replaceRoot: { newRoot: { coords: "$level1.level2.coordinates" } } }
)

As we know, in general, $project stage takes the field names and specifies 1 or 0/true or false to include the fields in the output or not, we also can specify the value against a field instead of true or false to rename the field. Below is the syntax

    db.test_collection.aggregate([
        {$group: {
            _id: '$field_to_group',
            totalCount: {$sum: 1}
        }},
        {$project: {
            _id: false,
            renamed_field: '$_id',    // here assigning a value instead of 0 or 1 / true or false effectively renames the field.
            totalCount: true
        }}
    ])