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Operating System:
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Steps To Reproduce:
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Given the following documents:

{ addresses: [{ city: 'Chicago' }, { city: 'London' }] } { addresses: [{ city: 'Seattle' }] } { addresses: [{ city: 'Chicago' }, { city: 'Seattle' }] }

And then I call $unwind in an aggregation pipeline:

{ "$unwind": "$addresses.city" } { "$group": { _id: "$addresses.city", counts: { "$sum": 1 } } } => {}

It will yield an empty result.

As a workaround, if I add a $project it will work:

{ "$project": { "p" => "$addresses.city" } } { "$unwind": "$p" } { "$group": { _id: "$p", counts: { "$sum": 1 } } } => { _id: "Chicago", counts: 2 } { _id: "London", counts: 1 } { _id: "Seattle", counts: 2 }
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Given the following documents: { addresses: [{ city: 'Chicago' }, { city: 'London' }] } { addresses: [{ city: 'Seattle' }] } { addresses: [{ city: 'Chicago' }, { city: 'Seattle' }] } And then I call $unwind in an aggregation pipeline: { "$unwind" : "$addresses.city" } { "$group" : { _id: "$addresses.city" , counts: { "$sum" : 1 } } } => {} It will yield an empty result. As a workaround, if I add a $project it will work: { "$project" : { "p" => "$addresses.city" } } { "$unwind" : "$p" } { "$group" : { _id: "$p" , counts: { "$sum" : 1 } } } => { _id: "Chicago" , counts: 2 } { _id: "London" , counts: 1 } { _id: "Seattle" , counts: 2 }

When using the AggregationPipeline, $unwind always gives an empty result with nested fields

For example:

{ "$unwind": "$addresses.city" }

Adding a $project stage will fix the issue:

{ "$project": { "p" => "$addresses.city" } }
{ "$unwind": "$p" }

Refer to Steps to Reproduce below for details.

duplicates

SERVER-59713 `$unwind` doesn't work on array subdocument fields