Checking for SQLAlchemy Model Instances

• SQLAlchemy uses a concept called Object Relational Mapping (ORM).
• You define classes that represent your database tables.
• Instances of these classes become objects that hold data from corresponding table rows.

Checking if an Object is a Model Instance:

There are two main approaches:

1. Using isinstance():

   • This is the recommended approach.
   • It checks if the object (obj) is an instance of a class derived from the base class used for SQLAlchemy models (DeclarativeBase).

   from sqlalchemy.ext.declarative import declarative_base
   
   Base = declarative_base()
   
   class User(Base):
       # ... your table definition
   
   my_user = User()  # Create a model instance
   
   if isinstance(my_user, User):
       print("my_user is an instance of the User model")
   

2. Using sqlalchemy.orm.class_mapper (not recommended):

   • This method is less common and considered internal to SQLAlchemy.
   • It attempts to get a mapper object associated with the class of the object. If it succeeds, the object is likely a model instance. However, this method might raise an exception in some cases.

   from sqlalchemy.orm import class_mapper
   
   try:
       class_mapper(my_user)
       print("my_user might be a model instance")
   except:
       print("my_user is not likely a model instance")
   

Choosing the Right Method:

• Use isinstance() for most cases. It's clear, reliable, and the recommended approach.
• Avoid relying on internal methods like class_mapper as they might change in future SQLAlchemy versions.

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class User(Base):
    # ... your table definition

my_user = User()  # Create a model instance

if isinstance(my_user, User):
    print("my_user is an instance of the User model")
else:
    print("my_user is not a User model instance")

1. Custom Attribute Check (Limited Use):

   • If your models share a specific custom attribute that wouldn't be present in other objects, you could check for its existence. However, this is not very reliable and tightly couples your code to the specific attribute.

   class User(Base):
       # ... your table definition
       is_model = True  # Custom attribute
   
   my_user = User()
   
   if hasattr(my_user, 'is_model'):
       print("my_user might be a model instance")  # Not guaranteed
   else:
       print("my_user is likely not a model instance")
   

   This approach is discouraged as it relies on a custom attribute and doesn't guarantee the object is a model instance.

2. Inspection with inspect.isclass (For Class Checks):

   • If you only need to check if a variable holds the class definition of a SQLAlchemy model, you can use the inspect module's isclass function along with checking if the class inherits from DeclarativeBase. This is not applicable to checking object instances.

   from sqlalchemy.ext.declarative import declarative_base
   import inspect
   
   Base = declarative_base()
   
   class User(Base):
       # ... your table definition
   
   if inspect.isclass(User) and issubclass(User, Base):
       print("User is a class derived from DeclarativeBase")