fix(index): preserve existing fields configuration (#13639) (#13640)

Co-authored-by: William Bouchard <46496014+willbouch@users.noreply.github.com>
Co-authored-by: Adrien de Peretti <adrien.deperetti@gmail.com>

.changeset/mean-rockets-worry.md

@@ -0,0 +1,5 @@
+---
+"@medusajs/index": patch
+---
+
+fix(index): preserve existing fields configuration (#13639)

packages/modules/index/src/utils/build-config.ts

@@ -845,7 +845,9 @@ function processEntity(
           ]
           intermediateEntityObjectRepresentationRef.moduleConfig =
             intermediateEntityModule
-          intermediateEntityObjectRepresentationRef.fields = ["id"]
+          if (!intermediateEntityObjectRepresentationRef.fields.includes("id")) {
+            intermediateEntityObjectRepresentationRef.fields.push("id")
+          }
 
           /**
            * We push the parent id only between intermediate entities but not between intermediate and link