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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Using  hybrid_property of SQLAlchemy leads to CompileError","articleBody":"### First Check\n\n- [X] I added a very descriptive title to this issue.\n- [X] I used the GitHub search to find a similar issue and didn't find it.\n- [X] I searched the SQLModel documentation, with the integrated search.\n- [X] I already searched in Google \"How to X in SQLModel\" and didn't find any information.\n- [X] I already read and followed all the tutorial in the docs and didn't find an answer.\n- [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).\n- [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).\n\n### Commit to Help\n\n- [X] I commit to help with one of those options 👆\n\n### Example Code\n\n```python\nfrom sqlalchemy.ext.hybrid import hybrid_property  \r\nfrom typing import Optional  \r\nfrom sqlmodel import SQLModel, Field, create_engine  \r\n  \r\nfrom datetime import datetime  \r\n                                    \r\n                                      \r\nclass ObjectTable(SQLModel, table=True):  \r\n    object_id: Optional[int] = Field(primary_key=True, default=None)  \r\n    first_detection_time: datetime = Field(index=True)  \r\n    last_detection_time: datetime = Field(index=True)  \r\n      \r\n    @hybrid_property                                    \r\n    def detection_time(self) -\u003e float:  \r\n        return (self.last_detection_time - self.first_detection_time).total_seconds()  \r\n      \r\n    class Config:                 \r\n        arbitrary_types_allowed = True  \r\n                                                                                       \r\n     \r\nif __name__ == \"__main__\":  \r\n    engine = create_engine(\"sqlite:///database.db\")  \r\n    SQLModel.metadata.create_all(engine)\n```\n\n\n### Description\n\nI am trying to create a hybrid property in an SQLModel class to allow more complex querying. Following the steps as described in the sqlalchemy docs here: https://docs.sqlalchemy.org/en/14/orm/extensions/hybrid.html I assumed that this would work and create a valid table. However, this code gives the error:\r\n `sqlalchemy.exc.CompileError: (in table 'objecttable', column detection_time'): Can't generate DDL for NullType(); did you forget to specify a type on this Column?`\r\nAt first, I assumed that a type hint was missing so I added the `float` return type to the `hybrid_property`. I am not sure what the problem is exactly but I assumed that this code would yield a valid table.\n\n### Operating System\n\nLinux\n\n### Operating System Details\n\nUbuntu 20.04\n\n### SQLModel Version\n\n0.0.6\n\n### Python Version\n\nPython 3.8.10\n\n### Additional Context\n\n_No response_","author":{"url":"https://github.com/deZakelijke","@type":"Person","name":"deZakelijke"},"datePublished":"2022-04-14T11:27:07.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/299/sqlmodel/issues/299"}