Aggregate Mongo queries accept multiple steps, including SQL-style joins using $lookup. This will add a new property to the results, with an array of documents from the other collection. You can then use $unwind to make each result map to a single document from the other collection. Finally, $project can help you select only the fields you care about.

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You can't directly export an "aggregate" Mongo to CSV using the mongoexport command, as it's intended for simpler data exports. However, there is a way to output the results of one such query to a new collection, which you can then export.

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You can find a document, update and return it using a single method: findAndModify. This is useful to deal with concurrency issues when multiple clients are trying to process documents from a collection and you don't want two of them processing the same one.

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Object IDs in Mongo store several variables (screenshot from MongoChef), including the time it was created, a machine identifier, a process ID and an internal counter.

Screenshot showing an object ID and its parsed values

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