Solving the Mysterious Error: “Backend DB : ‘str’ object has no attribute ‘_sa_instance_state'”

Have you ever encountered the frustrating error “Backend DB : ‘str’ object has no attribute ‘_sa_instance_state'” while working with SQLAlchemy and Python? You’re not alone! This pesky error can send even the most seasoned developers down a rabbit hole of confusion and desperation. Fear not, dear reader, for we’re about to embark on a journey to conquer this beast and emerge victorious!

What is the ‘_sa_instance_state’ attribute?

Before we dive into the solution, let’s take a step back and understand what this elusive attribute is. In SQLAlchemy, the ‘_sa_instance_state’ attribute is an internal property of an object that indicates it’s a SQLAlchemy-mapped object. This attribute is used by SQLAlchemy to track the state of an object, including its relationships, primary key, and other crucial information.

The Culprit: Passing Strings Instead of Objects

Now that we know what ‘_sa_instance_state’ is, let’s examine the most common cause of this error. Are you guilty of passing strings instead of objects to your database? Perhaps you’re using a function like `session.query(BackendDB).filter_by(id=”some_id”)`, where “some_id” is a string? This is precisely where things go awry!

Why Passing Strings is a No-Go

When you pass a string to a SQLAlchemy function, it doesn’t magically become an object. Instead, SQLAlchemy treats it as a literal string, which, of course, has no ‘_sa_instance_state’ attribute. This results in the error we’re trying to solve.

Solution 1: Pass Objects, Not Strings

The simplest solution is to pass the actual object instead of its string representation. For example:

from yourapp.models import BackendDB

some_id = "some_id"
obj = session.query(BackendDB).filter_by(id=some_id).first()
do_something_with_obj(obj)

In this example, we first retrieve the object using the `session.query` method and then pass the object to the `do_something_with_obj` function. Voilà! No more ‘_sa_instance_state’ errors!

Solution 2: Use SQLAlchemy’s `orm.aliased` Function

Sometimes, passing objects isn’t an option, and you need to work with strings. In such cases, SQLAlchemy’s `orm.aliased` function can be your savior.

What is `orm.aliased`?

`orm.aliased` is a SQLAlchemy function that creates an alias for a mapped class, allowing you to refer to it using a string. When you use `orm.aliased`, SQLAlchemy creates a temporary instance of the class, which has the ‘_sa_instance_state’ attribute we so desperately need.

Example Usage

from sqlalchemy import orm

aliased_obj = orm.aliased(BackendDB, name="aliased_backend_db")
result = session.query(aliased_obj).filter_by(id="some_id").first()
do_something_with_obj(result)

In this example, we create an alias for the `BackendDB` class using `orm.aliased`. We then use this alias to query the database and retrieve the object. Ta-da! The ‘_sa_instance_state’ error is gone!

Solution 3: Use SQLAlchemy’s `inspect` Function

Another approach is to use SQLAlchemy’s `inspect` function, which allows you to inspect the attributes of an object without having to load it from the database.

Example Usage

from sqlalchemy import inspect

some_id = "some_id"
obj = BackendDB()
obj.id = some_id
inspector = inspect(obj)
do_something_with_inspector(inspector)

In this example, we create a new `BackendDB` object and set its `id` attribute to the desired value. We then use the `inspect` function to create an inspector object, which provides access to the object’s attributes, including ‘_sa_instance_state’!

Conclusion

The error “Backend DB : ‘str’ object has no attribute ‘_sa_instance_state'” can be frustrating, but with these three solutions, you’ll be well-equipped to tackle it head-on. Remember, when working with SQLAlchemy, it’s essential to understand the difference between strings and objects. By avoiding the pitfalls of passing strings and instead using objects or clever workarounds, you’ll be able to overcome this error and write more robust, efficient code.

Solution	Description
Pass Objects, Not Strings	Pass the actual object instead of its string representation.
Use SQLAlchemy’s `orm.aliased` Function	Create an alias for a mapped class, allowing you to refer to it using a string.
Use SQLAlchemy’s `inspect` Function	Inspect the attributes of an object without having to load it from the database.

Now, go forth and conquer those SQLAlchemy errors like a boss!

• SQLAlchemy Official Documentation
• Stack Overflow: SQLAlchemy object has no attribute ‘_sa_instance_state’
• SQLAlchemy Google Group: ‘_sa_instance_state’ error

Happy coding!

Frequently Asked Question

Stuck with the error “Backend DB : ‘str’ object has no attribute ‘_sa_instance_state'”? Don’t worry, we’ve got you covered! Here are some frequently asked questions to help you troubleshoot the issue: