db.foo.aggregate([

        {

            $group: {

                "_id": "$_id",

                "contents": { $push: "$contents" },

                users: { $first: "$users" },

                owners: { $first: "$owners" },

                name: { $first: "$name" },

                category: { $first: "$category" },

                course: { $first: "$course" },

                // and remaining fields

            }

        }

    ])