Entity Framework Code First: Relationship mapping
I'm developing an Entity Framework Code First (v. ) C# library omarcafini.info Framework I don't know how to set zero-to-one relationship. By default, Entity Framework uses the Code First conventions to map your InstructorID); // Map one-to-zero or one relationship modelBuilder. Configuring one-to-zero or one relationships with Code First Model. Description. Database Diagram: Entity Framework One-To-Zero or One.
To enforce that every person can have only one car, PersonId would have to be unique in Car. Not to mention the case where you specify both ends of the relationship The only real way to enforce this rule if the People and the Car tables have the 'same' primary key same values in the connected records.
And this makes the whole schema a mess. Still, you have to be alert when using this solution, because it goes against the usual naming conventions, which might lead you astray.
Here's the schema generated from this model: So this relationship is not enforced by the database schema, but by Entity Framework itself.
That's why you have to be very careful when you use this, not to let anybody temper directly with the database.
Set foreign key to a zero-to-one relationship in Entity Framework Code First
Mapping one-to-one exactly Mapping one-to-one when both sides are required is also a tricky thing. Let's imagine how this could be represented with foreign keys. Now what happens if you want to insert a Car record? In order for this to succeed, there must be a PersonId in Car, because it is required. And for this PersonId to be valid, the corresponding record in People must exist. OK, so let's go ahead and insert the person record.
But for this to succeed, a valid CarId must be in the person record — but that car is not inserted yet! It cannot be, because we have to insert the referred person record first. But we cannot insert the referred person record, because it refers back to the car record, so that must be inserted first foreign key-ception: So this cannot be represented the 'logical' way either.
Again, you have to drop one of the foreign keys. Which one you drop is up to you. The side that is left with a foreign key is called the 'dependent', the side that is left without a foreign key is called the 'principal'.
Configure One-to-One relationship in Code First Entity Framework
And again, to ensure the uniqueness in the dependent, the PK has to be the FK, so adding an FK column and importing that to your model is not supported. So here's the configuration: That's because again, this is not enforced by the schema, but by EF itself.C# Entity Framework 6 - Database First - Part 2 - Foreign Keys & Relationships
So again, be careful: Mapping one or zero-to-one or zero And to finish off, let's briefly look at the case when both sides are optional. I just realized that this post has gotten way longer than I had anticipated: So I'm not going into the details and play with the idea of having two FK-s and the potential problems and warn you about the dangers of not enforcing these rules in the schema but in just EF itself. Here's the config you need to apply: Closing arguments Again, this post is way longer than it should be: And I don't have any ground-breaking wisdom on the topic anyway.
Configure One-to-Zero-or-One Relationship using Data Annotation Attributes Here, we will apply data annotation attributes on the Student and StudentAddress entities to establish a one-to-zero-or-one relationship.
The Student entity follows the default code-first convention as it includes the StudentId property which will be the key property. So, we don't need to apply any attributes on it because EF will make the StudentId column as a PrimaryKey in the Students table in the database. The StudentAddressId property follows the default convention for primary key.
One-To-Zero or One relationship
So, we don't need to apply any attribute for PK. However, we also need to make it a foreign key which points to StudentId of the Student entity.
So, apply [ForeignKey "Student" ] on the StudentAddressId property which will make it a foreign key for the Student entity, as shown below. Student includes the StudentAddress navigation property and StudentAddress includes the Student navigation property.
You can then configure foreign key properties by using the HasForeignKey method. This method takes a lambda expression that represents the property to be used as the foreign key.
The OfficeAssignment has the InstructorID property that is a primary key and a foreign key, because the name of the property does not follow the convention the HasKey method is used to configure the primary key.
However, when both ends of the relationship are required or both sides are optional Entity Framework cannot identify the dependent and principal. Instructor ; Configuring a Many-to-Many Relationship The following code configures a many-to-many relationship between the Course and Instructor types. In the following example, the default Code First conventions are used to create a join table. Courses If you want to specify the join table name and the names of the columns in the table you need to do additional configuration by using the Map method.