addToSet
Gets a field name for a $group operation representing all unique values that results from applying the given expression to each document in a group of documents that share the same group by key.
Return
the field
Parameters
expression
the expression
Creates an update that adds the given value to the array value of the property, unless the value is already present, in which case it does nothing
Return
the update
Parameters
property
the property
value
the value