AttributeError：“str”对象没有属性“”_sa_instance_state“”- Flask SqlAlchemy

AttributeError: 'str' object has no attribute '_sa_instance_state' is a common error in Flask SQLAlchemy when trying to access the attribute '_sa_instance_state' on a string object.

This error typically occurs when you are trying to perform database operations using Flask SQLAlchemy but mistakenly pass a string object instead of a SQLAlchemy model object. The '_sa_instance_state' attribute is automatically added to SQLAlchemy model objects and is used by SQLAlchemy to keep track of the object's state and manage database operations.

To resolve this error, you need to ensure that you are passing the correct SQLAlchemy model object to the database operations. Here are some steps you can follow to troubleshoot and fix this error:

1. Check your code: Review the code where you are encountering this error. Look for any places where you might be mistakenly passing a string object instead of a SQLAlchemy model object.
2. Verify the model object: Make sure that the object you are passing to the database operations is indeed a SQLAlchemy model object. Double-check the object's class and ensure that it inherits from the SQLAlchemy db.Model base class.
3. Check the function or method signature: If you are using Flask views or routes, check the function or method signature to ensure that you are correctly receiving the model object as a parameter. If necessary, update the function signature to expect the correct object type.
4. Review the database operations: Examine the database operations you are performing on the object. Ensure that you are using the correct methods and attributes specific to SQLAlchemy model objects. For example, if you are trying to save the object to the database, use the db.session.add() and db.session.commit() methods.
5. Test with a minimal example: If you are still unable to identify the issue, try creating a minimal example that reproduces the error. This can help isolate the problem and make it easier to find a solution.

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