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9 changes: 4 additions & 5 deletions source/reference/aggregation.txt
Original file line number Diff line number Diff line change
Expand Up @@ -404,16 +404,15 @@ The current pipeline operators are:
:pipeline:`$group` pipeline operators.

- If you specify a target field for :pipeline:`$unwind` that
does not exist in an input document, the document passes
through :pipeline:`$unwind` unchanged.
does not exist in an input document, the input document is
ignored (generates no result documents).

- If you specify a target field for :pipeline:`$unwind` that is
not an array, :mongodb:func:`aggregate()` generates an error.

- If you specify a target field for :pipeline:`$unwind` that
holds an empty array (``[]``), then that field is removed
from the result while all other fields are passed through
unchanged.
holds an empty array (``[]``) in an input document, the input
document is ignored (generates no result documents).

.. pipeline:: $group

Expand Down