signchange.js

var sign = (doc,fields) => {
   let toSign=""
   fields.forEach(f => { toSign += parseFloat(doc[f]) + "." } )
   toSign = toSign.replace(/NaN/,'.')
   print(`Signing: ${toSign}`)
   iv = 56781234
   for(c=0;c<toSign.length;c++) {
      v = parseInt(toSign[c])
      if(isNaN(v)) v=10; //Dots etc
      v += 1; //No point multiplying 0 by anything
print(v)
iv = (iv + ( 227 ** ((c+1)%6) * v)) % 1048576
}
return iv
}

doc = {_id: "test", x:1,y:1,s:5.5}
doc.cksum = sign(doc)

try { db.readonlyfields.insertOne( doc) } catch(e) {print(e.errmsg)}
{ "acknowledged" : true, "insertedId" : "test" }

try { db.readonlyfields.updateOne( {_id:"test"}, {$set:{b:2}}) } catch(e) {print(e.errmsg)}
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

try { db.readonlyfields.updateOne( {_id:"test"}, {$set:{x:2}}) } catch(e) {print(e.errmsg)}
Document failed validation