Compare two date fields in MongoDB

Question

in my collection each document has 2 dates, modified and sync. I would like to find those which modified > sync, or sync does not exist.

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Answer 1

For MongoDB 3.6 and newer:

The $expr operator allows the use of aggregation expressions within the query language, thus you can do the following:

db.test.find({ "$expr": { "$gt": ["$modified", "$sync"] } })

or using aggregation framework with $match pipeline

db.test.aggregate([
    { "$match": { "$expr": { "$gt": ["$modified", "$sync"] } } }
])

---

For MongoDB 3.0+:

You can also use the aggregation framework with the $redact pipeline operator that allows you to process the logical condition with the $cond operator and uses the special operations $$KEEP to "keep" the document where the logical condition is true or $$PRUNE to "remove" the document where the condition was false.

Consider running the following aggregate operation which demonstrates the above concept:

db.test.aggregate([
    { "$redact": {
        "$cond": [
            { "$gt": ["$modified", "$sync"] },
            "$$KEEP",
            "$$PRUNE"
        ]
    } }
])

This operation is similar to having a $project pipeline that selects the fields in the collection and creates a new field that holds the result from the logical condition query and then a subsequent $match, except that $redact uses a single pipeline stage which is more efficient: