In Entity Framework, the relationships between classes reflect how the table structure will be in the relational database.
Relationships between tables are a fundamental part of relational databases (that’s why they are called “relational” 😊). They allow you to organize and structure information.
In Entity Framework, relationships between tables are represented as relationships between entities.
Just like their database equivalents, we can have three fundamental types of relationships:
- One-to-one relationship (1:1): An entity A is related to a single entity B and vice versa.
- One-to-many relationship (1
): An entity A can be related to many B entities, but a B entity is only related to one A. - Many-to-many relationship (N
): Many A entities are related to many B entities and vice versa.
Let’s review each one, and we will see them in depth in their respective tutorial 👇.
One-to-One Relationship (1:1)
A one-to-one relationship occurs when a row in one table is related to only one row in another table.
- User - Profile (each user has a single profile).
- Employee - Computer (each employee has a single computer).
- Passport - Person (each passport belongs to a single person).
One-to-Many Relationship (1
A one-to-many relationship occurs when:
- An entity A can have many instances of an entity B
- But each instance of B is related to a single A
This is the most common type of relationship in databases.
- Publisher → Books (a publisher can publish many books, a book only has one publisher)
- Department → Employees (a department has many employees, an employee has one department)
- Customer → Orders (a customer places many orders, an order has one customer)
Many-to-Many Relationship (N
A many-to-many relationship occurs when:
- Multiple entities of class A can be associated with multiple entities of class B
- Multiple entities of class B can be associated with multiple entities of class A
This type of relationship is more complex due to the need for an intermediate table to store the associations.
- Students ←→ Courses (a student takes many courses, a course has many students)
- Products ←→ Orders (a product appears in many orders, an order contains many products)
- Users ←→ Roles (a user has many roles, a role belongs to many users)
