Return whole document from aggregation

Currently you cannot get the whole comment document via single $first operator. But you can include other necessary fields (similar to _id field) during $group step:

{
    "$group": {
        _id: "$post_id",
        lastComment: { "$first": "$_id" },
        field_1: { "$first": "$field_1" },
        field_2: { "$first": "$field_2" },
        // ...
        field_N: { "$first": "$field_N" }
    }
}

---

According to this JIRA ticket: https://jira.mongodb.org/browse/SERVER-5916, the whole document will be available to return from aggregation operations from 2.5.3 version. It will be possible using new variables: $$ROOT or $$CURRENT:

{
    "$group": {
        _id: "$post_id",
        lastComment: { "$first": "$$CURRENT" }
    }
}