How do you map objects to tables in ORM?

1 What is ORM mapping?

ORM mapping is the process of defining how your objects and their properties correspond to tables and columns in your database. For example, if you have a class called User in your code, you may want to map it to a table called users in your database, where each row represents a user and each column represents a property, such as name, email, or password. ORM mapping also involves specifying how your objects relate to each other, such as one-to-one, one-to-many, or many-to-many associations.

2 Why use ORM mapping?

ORM mapping has several benefits for developers. First, it abstracts away the differences between various database systems, so you can use the same code and syntax to interact with different databases. Second, it lets you use the features and paradigms of your programming language, such as inheritance, polymorphism, or validation, to manipulate data. Third, it reduces the amount of boilerplate code and SQL queries you have to write, making your code more readable and maintainable.

4 How to define ORM mapping?

To define ORM mapping, you need to use the conventions and annotations provided by your ORM framework. Conventions are the default rules and assumptions that the framework follows to map your objects to tables, such as using the same name for the class and the table, or using the primary key as the identifier. Annotations are the explicit declarations and configurations that you can add to your code to override or modify the conventions, such as specifying a different table name, column name, or data type, or defining the relationships and constraints between your objects.

5 What are the ORM mapping strategies?

When mapping objects to tables, there are three main ORM strategies to consider: table per class, table per subclass, and table per concrete class. The strategies vary in how they handle inheritance and polymorphism in the object model. Table per class is simple and flexible, but can lead to data duplication and inefficient queries. Table per subclass avoids data duplication and supports polymorphism, but can cause complex joins and constraints. Table per concrete class is efficient and simple, but does not facilitate polymorphism and may result in data duplication.

6 What are the ORM mapping challenges?

ORM mapping isn't a perfect solution and can pose some challenges and limitations for developers. These include the impedance mismatch, which is the difference between object-oriented and relational models of data. ORM mapping tries to bridge this gap, but it can't fully eliminate it and may require some compromises or trade-offs in design. Additionally, ORM mapping can introduce some overhead and complexity in code and queries, which can affect performance and efficiency. It can also hide details and logic of the database, making it harder to optimize and debug queries. Learning and mastering the syntax of your ORM framework, as well as the underlying database system, is necessary for successful ORM mapping. Additionally, you need to keep your ORM mapping up to date with code and database changes, which can require migrations and refactoring.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?