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    Type:Task 
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    Resolution: Fixed
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    Priority:Major - P3 
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    Affects Version/s: None
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    Component/s: None
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        Fully Compatible
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        Query 2019-10-21
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In the current mapReduce implementation, there is nothing in place to explicitly reject merging to an existing sharded collection whose shard key is not {_id: 1}. However, since it uses this upsert helper which always targets by _id, the command will eventually fail. We should keep this behavior in 4.4.
- depends on
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                    SERVER-42942 M/R Agg: Implement translation for cluster mapReduce command -         
- Closed
 
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- is depended on by
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                    SERVER-43829 Revisit mapReduce tests which were blacklisted from various passthoughs to see if they can be unblacklisted -         
- Closed
 
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