注解
The $each operator is only used with the $addToSet see the documentation of $addToSet for more information.
The $each operator is available within the $addToSet, which allows you to add multiple values to the array if they do not exist in the field array in a single operation. Consider the following prototype:
db.collection.update( { field: value }, { $addToSet: { field: { $each : [ value1, value2, value3 ] } } } );