db.a.update({_id:1}, {$push:{a:{$each:[]}}}, {upsert:1}) //creates a field
db.a.update({_id:1}, {$push:{a:{$each:[]}}}, {upsert:1}) // produces oplog entry
db.a.update({_id:1}, {$push:{a:{$each:[2,3]}}})
db.a.update({_id:1}, {$push:{a:{$each:[]}}}) // no oplog entry