Relationships - EF Core | Microsoft Docs
A relationship defines how two entities relate to each other. The following code listing shows a one-to-many relationship between Blog and Post .. for the join table and mapping two separate one-to-many relationships. C#. Entity Framework (EF) is an Object Relational Mapper (ORM) that can It will also create relationships between the tables if you have the. Learn how Entity Framework defines the relationship between entities in Entity The database includes the StudentCourse joining table which includes the.
The foreign key is defined in the table that represents the many end of the relationship. For example, in the above diagram Student and Enrollment tables have one-tomany relationship, each student may have many enrollments, but each enrollment belongs to only one student.
Relationships, navigation properties and foreign keys - EF6 | Microsoft Docs
In entity framework, these relationship can be created with code as well. Following is an example of Student and Enrollment classes which are associated with one to many relationship. Many-to-Many Relationship In many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa.
You can create such a relationship by defining a third table, called a junction table, whose primary key consists of the foreign keys from both table A and table B. For example, the Student and Course tables have many-to-many relationship that is defined by one-to-many relationship from each of these tables to the Enrollment table.
The following code contains the Course class and the above two classes, i. One-to-One Relationship In a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa.
A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.
Using the same example, I can set EquipmentType to be required by including an annotation when declaring it in the model class: By Convention So what is configuration by convention?
When you follow some simple rules on property types and names, Entity Framework will auto-configure many of the software and database features.
Configuring Relationships in Entity Framework Configuring scalar non-relational properties is quite straightforward, but when it comes to entity relationships, it can be more tricky to decide which approach is best for configuring them. By default, when I delete a Student, I will get a referential integrity error: So how do I fix this?
- Relationships in EF
- Configure a Many-to-Many Relationship using Fluent API
- One-to-Many Relationship
The official Microsoft documentation recommends manually deleting each foreign key for dependent entities: If we look at the documentation for Entity Framework 6 it says that: If a foreign key on the dependent entity is nullable, Code First does not set cascade delete on the relationship, and when the principal is deleted the foreign key will be set to null. Entity Framework should now set the foreign keys to null for these entities when I delete a student.
However as the code stands, I will still get the same referential integrity error.
The crucial detail here is that the dependent entities must be loaded before the principal entity is deleted. Here I explicitly load both the Equipmentand Transaction entities before removing the student entity: