test correct primary key inference

milli/src/update/index_documents/transform.rs

@@ -497,6 +497,7 @@ mod test {
     use super::*;
 
     mod compute_primary_key {
+
         use super::{compute_primary_key_pair, FieldsIdsMap};
 
         #[test]
@@ -537,5 +538,27 @@ mod test {
             assert!(result.is_err());
             assert_eq!(fields_map.len(), 0);
         }
+
+    }
+
+    mod primary_key_inference {
+        use bimap::BiHashMap;
+
+        use crate::update::index_documents::transform::find_primary_key;
+
+        #[test]
+        fn primary_key_infered_on_first_field() {
+            // We run the test multiple times to change the order in which the fields are iterated upon.
+            for _ in 1..50 {
+                let mut map = BiHashMap::new();
+                map.insert(1, "fakeId".to_string());
+                map.insert(2, "fakeId".to_string());
+                map.insert(3, "fakeId".to_string());
+                map.insert(4, "fakeId".to_string());
+                map.insert(0, "realId".to_string());
+
+                assert_eq!(find_primary_key(&map), Some("realId"));
+            }
+        }
     }
 }