Service Layer vs. Data Mapper: Who Should Handle Complex Query Conditions?

Determining Responsibility for Handling Conditions in Complex Queries: Service Layer vs. Data Mapper

In the pursuit of data retrieval and manipulation, the question arises: who should bear the burden of handling complex query conditions - the data mapper or the service layer?

The Data Mapper

The data mapper pattern dictates a simple interface, with key responsibilities being data fetching, saving, and deletion. However, its implementation details remain open to interpretation.

Despite its minimalist interface, the data mapper can be extended to incorporate some conditional logic, such as retrieving objects based on specific identifiers or author names.

The Service Layer

The service layer, on the other hand, acts as an intermediary between the controller and data mapper. It can alleviate the complexity of handling multiple conditions by calling more specific methods directly, such as BookDataMapper->getByAuthorAndPublisher().

Advantages of Service Layer Condition Handling

Some advocate for having the service layer parse query conditions for several reasons:

• Reduces complexity in the data mapper.
• Keeps conditional logic within the model layer, preventing its leakage to the controller.

Advantages of Data Mapper Condition Handling

Others prefer to centralize conditional logic within the data mapper:

• Simplifies the service layer, making it a mere intermediary.
• Allows the data mapper to incorporate additional condition-based methods as needed.

Optimal Approach

The optimal approach depends on the specific application and domain-specific logic. However, a few general guidelines can be considered:

• Keep the data mapper interface as simple as possible, with essential operations like fetch, save, and remove.
• Use the domain object itself to hold conditional parameters for data retrieval.
• Utilize the Tell Don't Ask principle to minimize communication between the domain object and mapper.
• Consider using additional structures like ArticleCollectionMapper to handle groups of objects with varying conditions.

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