Add support for custom fields in CMDT

Author: jamessimone

README.md

@@ -36,6 +36,12 @@ trigger ExampleTrigger on Opportunity(after insert, after update, before delete)
 
 That's it! Now you're ready to configure your rollups using Custom Metadata. `Rollup` makes heavy use of Entity Definition & Field Definition metadata fields, which allows you to simply select your options from within picklists, or dropdowns. This is great for giving you quick feedback on which objects/fields are available without requiring you to know the API name for every SObject and their corresponding field names.
 
+#### Special Considerations For Use Of Custom Fields As Rollup/Lookup Fields
+
+One **special** thing to note on the subject of Field Definitions — custom fields referenced in CMDT Field Definition fields are stored in an atypical way, and require the usage of additional SOQL queries as part of `Rollup`'s upfront cost. A typical `Rollup` operation will use `2` SOQL queries per rollup — the query that determines whether or not a job should be queued or batched, and a query for the specific Rollup Limit metadata (a dynamic query, which unfortunately means that it counts against the SOQL limits) — prior to going into the async context (where all limits are renewed) plus `1` SOQL qery (also dynamic, which is why it contributes even though it's querying CMDT). However, usage of custom fields as any of the four fields referenced in the `Rollup__mdt` custom metadata (details below) adds an additional SOQL query. If the SOQL queries used by `Rollup` becomes cause for concern, please submit an issue and we can work to address it!
+
+#### Rollup Custom Metadata Field Breakdown
+
 Within the `Rollup__mdt` custom metadata type, add a new record with fields:
 
 - `Calc Item` - the SObject the calculation is derived from — in this case, Oppportunity

rollup/main/default/classes/Rollup.cls

@@ -1010,6 +1010,7 @@ global without sharing virtual class Rollup implements Database.Batchable<SObjec
     return batchRollup;
   }
 
+  private static Map<String, String> durableIdToFieldName;
   private static SObjectField getSObjectFieldByName(DescribeSObjectResult objectDescribe, String desiredField) {
     Map<String, SObjectField> fieldNameToField = objectDescribe.fields.getMap();
     String paredFieldName = getParedFieldName(desiredField, objectDescribe);
@@ -1019,9 +1020,23 @@ global without sharing virtual class Rollup implements Database.Batchable<SObjec
     // for lookup fields, CMDT field-level definition fields store the field name, which is outrageous
     else if(fieldNameToField.containsKey(paredFieldName + 'Id')) {
       return fieldNameToField.get(paredFieldName + 'Id');
-    } else if(fieldNameToField.containsKey(paredFieldName + '__c')) {
-      return fieldNameToField.get(paredFieldName + '__c');
     }
+
+    try {
+      Id testCustomField = Id.valueOf(paredFieldName);
+      if(durableIdToFieldName == null) {
+        durableIdToFieldName = new Map<String, String>();
+        List<FieldDefinition> fieldDefinitions = [SELECT QualifiedApiName, DurableId FROM FieldDefinition WHERE EntityDefinitionId = : objectDescribe.getName()];
+        for(FieldDefinition fieldDef : fieldDefinitions) {
+          durableIdToFieldName.put(fieldDef.DurableId, fieldDef.QualifiedApiName);
+        }
+      }
+      String actualFieldName = durableIdToFieldName.get(desiredField);
+      return fieldNameToField.get(actualFieldName);
+    } catch(Exception ex) {
+      // do nothing, it didn't work
+    }
+
     return null;
   }