why do they use an empty field called "Field"
Because calculation rules expect a field to be calculated.
A very simple form of a calculation rule would be
// calculate field C as sum of fields A and B
var c = $feature.a + $feature.b
return c
In this simple form, you don't return a dictionary like in the blog post you linked, but a simple value. In the rule creation window you sspecify the field this value is stored in.
The advanced attribute rules enhance this behavior, so that by editing one table, you can do stuff in other tables. The basic behavior stays the same: the rule still wants to know what value to return and in which field to store the returned value.
In the blog post, they don't care about calculating attributes for the point, they just want to automatically create a polygon around the point (the inverse of what you are doing). But because the rule expects a return value and a field, they just use an empty field and return the value that is already there. You don't have to create an extra field for that. Just take a field that is already in your polygon FC (you have to be able to edit it, so not things like ObjectID or GlobalID) and return its value. By returning the value that is already stored in the field, you ensure that nothing is actually changed.
For eample, my calculation rules for primary keys look something like this:
// Calculation rule for a primary key field
// Field: BauwerkID
// Triggers: Insert, Update
// check if the field is empty
if(IsEmpty($feature.BauwerkID)) {
// yes -> return a new value
return NextSequenceValue("SequenceBauwerkID")
}
// no -> return the field's value (change nothing)
return $feature.BauwerkID
I need to add a "definition query ", am I on the right track with this approach?
//If value of field X is "A" or "B" proceed, else do nothing (Insert or Update).
if($feature.X == "A" || $feature.X == "B") {
// do stuff
}
else {
return $feature.Field // return the value that is already stored in your field. In this case, no point feature will be created.
}
// Personally, I find code easier to read if you do it the other way around:
if($feature.X != "A" && $feature.X != "B") {
return $feature.Field
}
// Do stuff
And how can I rewrite "A" to "C" and "B" to "D" at this point?
I'm not sure I understand you correctly: You have a polygon feature with X = "A", so you want to create a point feature and then change X to "C"?
In this case, your field for the calculation rule would be X.
// if field X is not "A" or "B", do nothing
if($feature.X != "A" && $feature.X !="B") {
return $feature.X
}
// else get the new value for X
var new_x = IIF($feature.X == "A", "C", "D") // $feature.X can only be "A" or "B" at this point (else we would have returned above)
// get the centroid
var centroid = ...
// return
return {
"result": new_x,
"edit": ...
}
Have a great day!
Johannes
