MongoDB projection result as an array of selected items?

In MongoDB, projection results as an array of selected items can be achieved using the distinct() method. This method returns an array of distinct values for a specified field from documents that match the given query criteria.

Syntax

db.collection.distinct(field, query)

Where field is the field to return distinct values from, and query is the optional filter criteria.

Sample Data

Let us create a collection with documents:

db.demo151.insertMany([
    {"ListOfNames": ["Chris", "David", "Mike"]},
    {"ListOfNames": ["Mike", "Bob"]},
    {"ListOfNames": ["John", "David", "Chris"]}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e3513b6fdf09dd6d08539da"),
        ObjectId("5e3513c4fdf09dd6d08539db"),
        ObjectId("5e3513dcfdf09dd6d08539dc")
    ]
}

View Sample Data

db.demo151.find();

{ "_id": ObjectId("5e3513b6fdf09dd6d08539da"), "ListOfNames": ["Chris", "David", "Mike"] }
{ "_id": ObjectId("5e3513c4fdf09dd6d08539db"), "ListOfNames": ["Mike", "Bob"] }
{ "_id": ObjectId("5e3513dcfdf09dd6d08539dc"), "ListOfNames": ["John", "David", "Chris"] }

Example: Get Array of Document IDs

To get an array of _id values from documents where the ListOfNames array contains "Mike":

db.demo151.distinct('_id', {'ListOfNames': "Mike"});

[
    ObjectId("5e3513b6fdf09dd6d08539da"),
    ObjectId("5e3513c4fdf09dd6d08539db")
]

More Examples

Get all distinct names from the ListOfNames arrays:

db.demo151.distinct('ListOfNames');

["Bob", "Chris", "David", "John", "Mike"]

Conclusion

The distinct() method efficiently projects results as an array of unique values from specified fields. It's particularly useful for extracting specific field values based on query conditions without retrieving entire documents.