5.36Building a lookup map in DataWeave

Suppose Salesforce returns:

[
  {
    "Id": "001A",
    "External_Customer_ID__c": "C001"
  },
  {
    "Id": "001B",
    "External_Customer_ID__c": "C002"
  }
]

Conceptually you want:

{
  "C001": "001A",
  "C002": "001B"
}

Then you can efficiently associate source records with Salesforce IDs.

This is where DataWeave tools such as:

map
mapObject
groupBy

become relevant.

You don't need to memorize the exact one-liner yet; understand the optimization strategy.