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Example function:
db.coll.insert({id: 'foo', a: 100, b: 200, c: 300, d: {e: 400, f:500}})
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db.coll.aggregate([
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{$docUnwind: {id: 0}}, // syntax similar to $project for whitelisting or blacklisting which fields to unwind
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])
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{id: 'foo', f_name: 'a', f_value: 100}
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{id: 'foo', f_name: 'b', f_value: 200}
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{id: 'foo', f_name: 'c', f_value: 300}
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{id: 'foo', f_name: 'd.e', f_value: 400}
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{id: 'foo', f_name: 'd.f', f_value: 500}
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In short, it will take the fields in a document, and unwind a portion of them into name, value pairs, making a separate document for each.
One use case for this is collecting statistics across a dynamic list of fields.
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