Updating many to many relationships in Entity Framework Core – The Reformed Programmer
Conventions for One-to-Many Relationships. There are certain conventions in Entity Framework which if followed in entity classes (domain classes) will. How to configure a many to many relationship in Entity Framework Core. In the books and categories example, this would result in a field for. Updating many to many relationships in Entity Framework Core an updated version of that article, but for Entity Framework Core (EF Core).
Relationships are tracked through object references instead of foreign key properties. This type of association is called an independent association. The most common way to modify an independent association is to modify the navigation properties that are generated for each entity that participates in the association.
You can choose to use one or both types of associations in your model. However, if you have a pure many-to-many relationship that is connected by a join table that contains only foreign keys, the EF will use an independent association to manage such many-to-many relationship.
The following image shows a conceptual model that was created with the Entity Framework Designer. The model contains two entities that participate in one-to-many relationship. Both entities have navigation properties.
Course is the depend entity and has the DepartmentID foreign key property defined.
The following code snippet shows the same model that was created with Code First. For information on setting up relationships in your model, see the following pages. Creating and modifying relationships In a foreign key association, when you change the relationship, the state of a dependent object with an EntityState. Unchanged state changes to EntityState. In an independent relationship, changing the relationship does not update the state of the dependent object. The following examples show how to use the foreign key properties and navigation properties to associate the related objects.
With foreign key associations, you can use either method to change, create, or modify relationships. With independent associations, you cannot use the foreign key property.
By assigning a new value to a foreign key property, as in the following example. DepartmentID; The following code removes a relationship by setting the foreign key to null. Note, that the foreign key property must be nullable.
Configuring Many To Many Relationships in Entity Framework Core
Synchronization does not occur because the object context does not contain permanent keys for added objects until they are saved. If you must have new objects fully synchronized as soon as you set the relationship, use one of the following methods.
The following code creates a relationship between a course and a department. If the objects are attached to the context, the course is also added to the department.
Courses collection, and the corresponding foreign key property on the course object is set to the key property value of the department.
If you are working with Entity Framework that is based on. You can also set the current value to null using the following method. For example, you can add an object of type Course to the department. This operation creates a relationship between a particular course and a particular department.
If the objects are attached to the context, the department reference and the foreign key property on the course object will be set to the appropriate department.
Many-to-many relationships in EF Core – Part 1: The basics « One Unicorn
Add newCourse ; By using the ChangeRelationshipState method to change the state of the specified relationship between two entity objects. By default, when I delete a Student, I will get a referential integrity error: So how do I fix this? 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: When the Equipment and Transaction entities are loaded into the working memory, EF knows to set the relevant dependent entities foreign keys to null.
This is a good example of letting the conventions work for you! The default Entity Framework is to set null the foreign key on a dependent entity when the principal entity is deleted. If I wanted to override the default delete behavior setting, for example to set the dependent entities to cascade on delete, then the Fluent API would be the place to configure that.