updateMask property
The fields to update in this write.
This field can be set only when the operation is update. If the mask is
not set for an update and the document exists, any existing data will be
overwritten. If the mask is set and the document on the server has fields
not covered by the mask, they are left unchanged. Fields referenced in the
mask, but not present in the input document, are deleted from the document
on the server. The field paths in this mask must not contain a reserved
field name.
Implementation
DocumentMask? updateMask;