Where should business logic live in a Django application?

Balances model methods, QuerySets, managers, and application services by the responsibility they own.

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TL;DR

I keep behavior that protects one entity's state close to the model, and reusable database selection or set-based operations in custom QuerySets or managers. A workflow that coordinates several models, authorization context, transactions, or external systems belongs in an explicit application service or use-case function. Views and serializers should translate HTTP input and output, then call that behavior rather than becoming the only place the rule exists.

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Practical judgment about cohesive behavior without fat-model dogma or ceremonial service layers.

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Overview
There is no single Django folder for all business logic; ownership follows the rule and its dependencies.
I keep behavior that protects one entity's state close to the model, and reusable database selection or set-based operations in custom QuerySets or managers. A workflow that coordinates several models, authorization context, transactions, or external systems belongs in an explicit application service or use-case function. Views and serializers should translate HTTP input and output, then call that behavior rather than becoming the only place the rule exists.
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The boundary is about cohesion, not line count. A model method is useful when callers should ask the entity to perform a valid transition. A QuerySet is useful when the operation is naturally about many rows and can stay in the database. A service earns its place when it gives a multi-step workflow one name, one transaction owner, and one testable entry point; it should not merely wrap every ORM call.

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Details

Putting every rule in views because they already have the request.
Using models as unstructured containers for unrelated workflows.
Creating one service class per model with only pass-through methods.

Where should business logic live in a Django application?

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