Relationship type annotations disappear after class definition is evaluated #530

Activity

1. phdowling

   added

   questionFurther information is requestedFurther information is requested

   on Jan 15, 2023
2. jarey commented on Dec 2, 2024

   

   jarey

   on Dec 2, 2024

   Last edited by jarey

   More actions

   I'm also facing this issue. From what I could see, one potential solution could be the following (if support for relationship information as part of the pydantic model is not included as a feature in SQLModel fure releases, so consider this a workaround):

   • Override the __new__ dunder method ( as __init__ is not used by SQLModel).
   • Populate the pydantic _model_fields property with missing info from relationships.
   • Rebuild pydantic model schema to reflect such changes.

   In your particular use-case it would be something like the following (behaviour could potentially be abstracted at SQLModel level, or at least at a base class level, so it is not performed in a per-class/per-attribute level each time):

   from sqlmodel import Relationship, SQLModel, Field
   from pydantic.main import BaseModel
   
   
   class SpecialFieldValidator(BaseModel):
       id: int = Field(primary_key=True, index=True)
   
       def __init_subclass__(cls) -> None:
           # Try to figure out what type the special_field relation has
           print("special_field" in cls.__annotations__)  # False
           print("special_field" in cls.__fields__)  # False
           print("special_field" in cls.__sqlmodel_relationships__)  # True, but no type info via this dict, just a plain RelationshipInfo() instance:
           print(dict(cls.__sqlmodel_relationships__["special_field"].__dict__))  # {'back_populates': None, 'link_model': None, 'sa_relationship': None, 'sa_relationship_args': None, 'sa_relationship_kwargs': None}
   
           return super().__init_subclass__()
   
   class MyModelA(SQLModel, SpecialFieldValidator, table=True):
       id: int = Field(primary_key=True)
       special_field_id: int = Field(foreign_key="mymodelb.id")
       special_field: "MyModelB" = Relationship()
   
       def __new__(cls, *args: Any, **kwargs: Any) -> Any:
           new_object = super().__new__(cls, *args, **kwargs)
           new_object.model_fields.update({"special_field": Field(..., description="description of your model property coming from the relationship")})
           new_object.model_rebuild(force=True)
           return new_object
   
   class MyModelB(SQLModel, SpecialFieldValidator, table=True):
       id: int = Field(primary_key=True)
       special_field_id: int = Field(foreign_key="mymodela.id")
       special_field: MyModelA = Relationship()
      
       def __new__(cls, *args: Any, **kwargs: Any) -> Any:
           new_object = super().__new__(cls, *args, **kwargs)
           new_object.model_fields.update({"special_field": Field(..., description="description of your model property coming from the relationship")})
           new_object.model_rebuild(force=True)
           return new_object

   Kind regards.

   👀React with 👀1Reacted by Manos Dimitrakis
3. aliyevr889

   added a commit that references this issue on Apr 11, 2026

   test: add regression tests for issue fastapi#530 - relationship annot…

   ...

   b430244
4. fastapi

   locked and limited conversation to collaborators on May 18, 2026
5. tiangolo

   converted this issue into a discussion #1971 on May 18, 2026