How do I update the content selected from a dropdown cell if the source subsequently changes? (Insert cell reference instead of literal value.) - google-sheets

Question

I have two sheets used to track a construction project.
On the first sheet, a list of tasks is incorporated into a timeline with cost projections, etc. The tasks are something like the following:
Cut Concrete
Pour new pad
Frame
Roof
The second sheet is for tracking individual purchases, each of which is associated with a task from the first sheet (e.g., Cut Concrete). It looks something like the following:
DATE PAYEE ITEM CATEGORY COST
----- ---------- ---------- -------- ------
10/25 Home Depot (10) 2x4's Frame ▽ $54.00
Using Data Validation, the Category dropdown in the second sheet references the list of items from the first sheet. This is working perfectly. Here's the problem...
If I change the item on the first sheet (for instance, "Frame" to "Framing"), although the dropdown is updated, any previously entered rows (such as the one shown above), just show a validation error (i.e., a red indicator in the right corner of the cell).
Since a construction project can easily have hundreds of items purchased, rather manually looking for data validation errors, is there a way to have the second sheet's values updated? For instance...
Add a script that watches for content changes in the first sheet. When the user starts editing a "task" cell, its original value is noted; and upon exiting the cell, if the value has changed, the script looks through the second spreadsheet for the original value and replaces it with the new one. (Seems like a lot of hassle.)
Find some way for the dropdown to insert a cell reference to the first sheet instead of the actual value. That way, the dropdown cell is always referencing the source item (i.e., the "task" cell's current content).
A more obvious feature I don't know about.

Answer 1

As per my favored possibility noted in my question, I figured out how to write a script that would run after a value was selected from the dropdown, thus overwriting the literal value with a cell reference.
The following script runs after the user makes a selection in the dropdown menu:
function onEdit(event){
var activeSheet = event.source.getActiveSheet();
var activeSheetName = activeSheet.getName();
var activeCell = activeSheet.getActiveCell();
var activeColumn = activeCell.getColumn();
if (activeSheetName == "Envelope (Spent)" && activeColumn === 4) {
var destinationCell = activeCell;
var destinationContent = destinationCell.getValue();
var sourceSheet = SpreadsheetApp.getActiveSpreadsheet().getSheets()[0];
var sourceSheetName = sourceSheet.getName();
var sourceRange = sourceSheet.getRange("D:D");
var sourceValues = sourceRange.getValues();
for (var i = 0; i < sourceValues.length; i++) {
if (sourceValues[i] == destinationContent) {
var sourceRow = sourceSheet.getRange("D" + i + ":D" + i).getRow() + 1;
destinationCell
.setValue("='" + sourceSheetName + "'!D" + sourceRow)
.setNote(destinationContent)
;
}
}
}
}
To allow for an easy recovery in case the two sheets somehow get out of sync, the originally selected value, derived from the dropdown's data validation, is inserted as a note. I figured it was easier to clear all the notes in the future than to find myself with a bunch of entries that don't correspond with the source list of tasks.

Answer 2

The data validation in Google sheets is always inserting the Literals you type or pick so you can't link it to the original cell. You have to solve the whole thing through scripts
Get a list of all data validation items through onOpen()
Create an onEdit() function that runs if the data validation range is edited, checks which field is changed and then goes through the purchases, checks which purchases have the old value and replaces them with the new one.