How to Handle Complex Query Conditions: Data Mapper vs. Service Layer

When dealing with complex query conditions in object-oriented programming, there are two main approaches: handling them in the data mapper or in the service layer.

Data Mapper Approach

The data mapper pattern is used to map domain objects to and from a database. In the context of handling conditions, the data mapper can have a generic get() method that accepts a list of conditions. For example, if we want to retrieve books by author, we could call BookDataMapper->get(['author' => 'John Doe']).

However, if we want to use multiple conditions that require a complex query, such as finding books by author and publisher, we could expand the BookDataMapper->get() method to parse out the conditions. Alternatively, we could create a new method like BookDataMapper->getByAuthorAndPublisher().

Service Layer Approach

The service layer is responsible for handling the business logic of the application. It can act as an intermediary between the controller and the data mapper. In the case of handling complex query conditions, the service layer could call the more specific data mapper methods directly, such as BookService->getByAuthorAndPublisher(). This approach would reduce the service layer to merely a middle-man.

Comparison of Approaches

The data mapper approach keeps the conditions within the data mapper, which can lead to conditional logic leaking out of the domain objects. On the other hand, the service layer approach keeps the domain logic within the service layer, making it more modular.

Ultimately, the choice between these approaches depends on the specific requirements of the application. If the domain logic is relatively simple and the complex queries are infrequent, the data mapper approach may be sufficient. However, if the domain logic is complex or the complex queries are frequent, the service layer approach may be more appropriate.