diff --git a/fastcrud/crud/fast_crud.py b/fastcrud/crud/fast_crud.py
index f813407..e2cc37b 100644
--- a/fastcrud/crud/fast_crud.py
+++ b/fastcrud/crud/fast_crud.py
@@ -918,10 +918,16 @@ class FastCRUD(
                 else None
             )
 
+            # Determine effective prefix for column selection to avoid conflicts
+            # If no explicit prefix provided, use table name + "_" to ensure uniqueness
+            effective_prefix = join.join_prefix
+            if effective_prefix is None:
+                effective_prefix = join.model.__tablename__ + "_"
+            
             join_select = _extract_matching_columns_from_schema(
                 model=join.model,
                 schema=join.schema_to_select,
-                prefix=join.join_prefix,
+                prefix=effective_prefix,
                 alias=join.alias,
             )
 
@@ -1240,10 +1246,16 @@ class FastCRUD(
                 else None
             )
 
+            # Determine effective prefix for column selection to avoid conflicts
+            # If no explicit prefix provided, use table name + "_" to ensure uniqueness
+            effective_prefix = join.join_prefix
+            if effective_prefix is None:
+                effective_prefix = join.model.__tablename__ + "_"
+            
             join_select = _extract_matching_columns_from_schema(
                 model=join.model,
                 schema=join.schema_to_select,
-                prefix=join.join_prefix,
+                prefix=effective_prefix,
                 alias=join.alias,
             )
 
diff --git a/fastcrud/crud/helper.py b/fastcrud/crud/helper.py
index ad25fbf..c8f0449 100644
--- a/fastcrud/crud/helper.py
+++ b/fastcrud/crud/helper.py
@@ -108,9 +108,18 @@ def _nest_join_data(
     for key, value in data.items():
         nested = False
         for join in join_definitions:
-            if join.join_prefix and key.startswith(join.join_prefix):
-                nested_key = join.join_prefix.rstrip("_")
-                nested_field = key[len(join.join_prefix) :]
+            # Determine what prefix was used for this join during column selection
+            if join.join_prefix is not None:
+                # Explicit prefix was provided
+                prefix_to_check = join.join_prefix
+                nested_key = join.join_prefix.rstrip("_") if join.join_prefix else join.model.__tablename__
+            else:
+                # No explicit prefix, we used table_name + "_" as effective prefix
+                prefix_to_check = join.model.__tablename__ + "_"
+                nested_key = join.model.__tablename__
+                
+            if prefix_to_check and key.startswith(prefix_to_check):
+                nested_field = key[len(prefix_to_check) :]
                 if nested_key not in nested_data:
                     nested_data[nested_key] = {}
                 nested_data[nested_key][nested_field] = value