[Solved]-Import RelatedManager from django.db.models.fields.related

The related manager classes are created at runtime inside generator functions in django.db.models.fields.related thus you can’t import them directly. If you want to check if an object is a related manager for a specific relation you can use isinstance(obj, MyModel.my_relation.__class__). You could also use hasattr to determine if the object has the properties you need (ducktyping) and avoid using isinstance altogether.