MapAttribute in mixin fails to initialize attr_name when used in PynamoDB models

[AleMazzari]

Description:
When defining a mixin that provides a nested attribute via MapAttribute, PynamoDB does not properly initialize the attribute metadata (attr_name, attr_path) unless the mixin itself is processed by PynamoDB’s metaclass. As a result, importing a model that inherits both Model and the mixin raises an AttributeError about missing attr_name.

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Steps to Reproduce:

1. Define your nested attribute

   from pynamodb.attributes import MapAttribute, UnicodeAttribute
   
   class CreatedByAttribute(MapAttribute):
       id          = UnicodeAttribute(null=False)
       given_name  = UnicodeAttribute(null=False)
       family_name = UnicodeAttribute(null=False)
       email       = UnicodeAttribute(null=False)

2. Define the mixin without metaclass

   from pynamodb.attributes import AttributeContainerMeta
   
   class AuditMixin(metaclass=AttributeContainerMeta):
       created_by = CreatedByAttribute(null=True)

3. Use the mixin in your model

   from pynamodb.models import Model
   from chalicelib.models.mixins import AuditMixin
   
   class Company(Model, AuditMixin):
       class Meta:
           table_name = "company"
       pk = UnicodeAttribute(hash_key=True)
       sk = UnicodeAttribute(range_key=True)

4. Import the model (e.g. in your Chalice app.py)

   from chalicelib.models.company.company import Company

5. Observe the error on import

   AttributeError: 'CreatedByAttribute' has no attribute 'attr_name'. Did you mean: 'attr_type'?
   

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Expected Behavior:

• PynamoDB should recognize CreatedByAttribute as a MapAttribute, initialize its metadata (attr_name, attr_path) when the mixin is defined, and allow models to inherit that nested attribute without error.

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Actual Behavior:

• Because the mixin is not processed by PynamoDB’s AttributeContainerMeta at the time its attributes are defined, the nested MapAttribute lacks attr_name. When the Model metaclass runs, it calls _initialize_attributes, encounters the missing property, and raises an AttributeError.

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Root Cause:

• PynamoDB only initializes attribute metadata (including attr_name) when a class is created with its internal metaclass (AttributeContainerMeta), which normally happens for Model and for subclasses of MapAttribute declared directly on a class using that metaclass. Mixins without that metaclass skip this initialization.

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Workarounds / Proposed Solution:

1. Ensure mixins use the correct metaclass so that all nested attributes are initialized up-front:

   class AuditMixin(metaclass=AttributeContainerMeta):
       created_by = CreatedByAttribute(null=True)

2. Extract mixins into a separate module that is imported before any models, to guarantee the metaclass runs before model definitions.

3. (Library change) Consider enhancing PynamoDB so that any MapAttribute encountered during model construction—regardless of whether it came via mixin—triggers attribute initialization automatically.

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Environment:

• PynamoDB version: e.g. 2023.2.0
• Python version: e.g. 3.12
• Operating System: macOS (Apple Silicon / Intel)

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Additional Notes:

• This only affects attributes provided via mixins (or intermediate base classes) that are not themselves subclasses of PynamoDB’s AttributeContainer or declared with the PynamoDB metaclass.
• Directly declaring MapAttribute fields on a Model works as expected.