Aggregation fields should allow aggregation expression to be assigned to nested field. [DATAMONGO-2353]

**[Christoph Strobl](https://jira.spring.io/secure/ViewProfile.jspa?name=cstrobl)** opened **[DATAMONGO-2353](https://jira.spring.io/browse/DATAMONGO-2353?redirect=false)** and commented


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**Reference URL:** https://stackoverflow.com/questions/57614132/project-sub-document-with-conditionaloperators-applied-spring-mongo-data
