What are the Three Key Structures of the Model Layer in MVC?

Understanding the Structure of a Model in MVC

Contrary to common misconception, the model in MVC is a layer, not a single class or object. This layer consists of three main structures:

1. Domain Objects

• Represent logical entities in the problem domain
• Define business logic for validating, computing, etc.
• Unconcerned with storage

2. Data Mappers

• Responsible for persistence
• May interact with databases, XML files, or other storage mechanisms
• Provide an abstraction over the physical storage layer

3. Services

• Provide a higher-level interface to domain business logic
• May interact with both domain objects and data mappers
• Can encapsulate common logic and interactions, reducing complexity in controllers

Accessing the Model

Controllers and views should access model services through dependency injection or a factory to ensure loose coupling and testability.

Interacting with the Model

• Controllers obtain user input and alter the model state based on it.
• Controllers should not validate input or perform database queries.
• Views display the current model state to the user.

Building the Model

Focus on creating individual services that implement specific functions, rather than trying to build a single "Model" class. Then, create domain objects and data mappers as needed.

Additional Considerations

• Database Tables and the Model: The relationship between database tables and model components may not be 1:1:1 in all cases.
• Views are not Templates: Views are responsible for presentational logic, using templates to assemble content based on data obtained from the model.
• View-Controller Relationship: In web development, it's typically preferred to have a 1:1 relationship between views and controllers, with each controller handling requests for a specific page.

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