{"foo" : "abc-123", more fields.... }
{"foo" : "abc-456", more fields.... }
{"foo" : "cde-000", more fields.... }
{"foo" : "cde-555", more fields.... }
{"foo" : "else-9991234", more fields.... }

{
    "abc": 2,
    "cde": 2,
    "else": 1
}

myColl.aggregate([
     $project: { firstPart: { ???? '$foo'

db.collection.find().forEach(function(el){
  // here split your el['foo'] and update the el with new `foo1` which will have your first part
})