sqlmodel ValueError: <class 'list'> has no matching SQLAlchemy type

Question

Given these classes and 1-to-N relationships, I get compile time error: ValueError: <class 'list'> has no matching SQLAlchemy type referring to the category.py field todos: list["Todo"], while the relation hero/team does work.

I also tried to replace the UUID type and use int in both classes, without success.

# hero.py (works)
class Hero(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    ....

    team_id: int | None = Field(default=None, foreign_key="team.id")
    team: Team | None = Relationship(back_populates="heroes")


# team.py (works)
class Team(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    .....

    heroes: list["Hero"] = Relationship(back_populates="team")


# todo.py (does not work)
class Todo(SQLModel, table=True):

    id: UUID | None = Field(
        default=uuid4(),
        primary_key=True,
    )

    ......

    category_id: UUID | None = Field(default=None, foreign_key="category.id")
    category: Category | None  = Relationship(back_populates="todos")


# category.py  (does not work => ValueError: <class 'list'>)
class Category(SQLModel, table=True):
    id: UUID | None = Field(default=uuid4(), primary_key=True)
    .....

    todos: list["Todo"] = Relationship(back_populates="category")

Full error log:

import todo.models.category
  File ".../todo/models/category.py", line 9, in <module>
    class Category(SQLModel, table=True):
  File ".../lib/python3.12/site-packages/sqlmodel/main.py", line 559, in __new__
    col = get_column_from_field(v)
          ^^^^^^^^^^^^^^^^^^^^^^^^
  File "../lib/python3.12/site-packages/sqlmodel/main.py", line 708, in get_column_from_field
    sa_type = get_sqlalchemy_type(field)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "../lib/python3.12/site-packages/sqlmodel/main.py", line 697, in get_sqlalchemy_type
    raise ValueError(f"{type_} has no matching SQLAlchemy type")
ValueError: <class 'list'> has no matching SQLAlchemy type

Answer 1

I encountered this issue when I tried to define field with list[str] type, like this:

class Example(SQLModel):
    labels: list[str] | None = None

and solved it by making the field store data in JSON by using Column and JSON from sqlalchemy:

class Example(SQLModel):
    labels: list[str] | None = Field(default=None, sa_column=Column(JSON))