Add custom relationship statements

sqladmin/forms.py

@@ -1,6 +1,7 @@
 """
 The converters are from Flask-Admin project.
 """
+
 from __future__ import annotations
 
 import enum
@@ -23,6 +24,7 @@
     RelationshipProperty,
     sessionmaker,
 )
+from sqlalchemy.sql import Select
 from sqlalchemy.sql.elements import Label
 from sqlalchemy.sql.schema import Column
 from wtforms import (
@@ -109,6 +111,11 @@ def _inner(func: T_CC) -> T_CC:
 
 
 class ModelConverterBase:
+    relationships_statements: dict[str, Select] = {}
+    """
+    Select statement for relationships.
+    """
+
     _converters: dict[str, ConverterCallable] = {}
 
     def __init__(self) -> None:
@@ -229,7 +236,10 @@ async def _prepare_select_options(
         session_maker: sessionmaker,
     ) -> list[tuple[str, Any]]:
         target_model = prop.mapper.class_
-        stmt = select(target_model)
+        if prop.key in self.relationships_statements:
+            stmt = self.relationships_statements[prop.key]
+        else:
+            stmt = select(target_model)
 
         if is_async_session_maker(session_maker):
             async with session_maker() as session: