Configure One-to-One relationship in Code First Entity Framework
This tutorial shows how to configure many-to-many relationship between entities using EF 6 includes default conventions for many-to-many relationships. The following is the context class that includes the Student and Course entities. The Map() method takes Action type delegate, hence, we can pass the lambda. Entity Framework Code First relationship mapping (One-to-many, My first example, where everybody can have any number of cars (meaning. Here's an example from the Programming E.F. Code First book . one-to-many relationship between LoyaltyUserDetail and PIIUser so you mapping should be.
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. ToTable "CourseInstructor" ; m. By convention, Code First always interprets a unidirectional relationship as one-to-many. For example, if you want a one-to-one relationship between Instructor and OfficeAssignment, where you have a navigation property on only the Instructor type, you need to use the fluent API to configure this relationship.
If a foreign key on the dependent entity is not nullable, then Code First sets cascade delete on the relationship. You already have your relationship set up. In the database, this is represented with foreign keys, of course. This model already supports a person not having a car, but it doesn't support a car without a person.Part 22 Many to many relationship in entity framework
To change this, simply change the type of PersonId to int?. Note that the PersonId column is now nullable in the Cars table. How does EF know which column is the foreign key and where should it point to? The conventions, of course.
Fluent API - Relationships
Having a property of type Person that is named Person with a PersonId property leads EF to conclude that PersonId is a foreign key, and it points to the primary key of the table represented by the type Person.
No worries; you can help EF with some hints about your relationships and keys in the model. Create an entity type configuration and apply it in your OnModelCreating: And finally the property representing the foreign key is specified HasForeignKey. This gives us the schema we want: You could configure the relationship from the Person side as well: Doesn't matter if you configure the relationship from the Person side or the Car side. You can even include both, but in this case be careful to specify the same relationship on both sides!
In this case be careful to specify the column of the foreign key as a nullable type! Mapping many-to-many Let's move on to the other scenario, where every person can have multiple cars and every car can have multiple owners. This is a many-to-many relationship. The easiest way is to let EF do it's magic using conventions.
Just change the model like this: As you can see, EF recognized the need for a join table, where you can keep track of person-car pairings. Quite simple, isn't it? You might want to rename the fields in the join table to be a little more friendly. You can do this by using the usual configuration methods again, it doesn't matter which side you do the configuration from: MapLeftKey "OwnerId" ; m.
Fluent API - Configuring and Mapping Properties and Types
MapRightKey "CarId" ; m. And this gives you exactly that schema: I have to admit, I'm not really a fan of this solution.
You cannot track extra information to a person-car association let's say the date from which it is validbecause you can't modify the table. Also, the CarId in the join table is part of the primary key, so if the family buys a new car, you have to first delete the old associations and add new ones.
In this case what you can do is create a join entity that has a reference to both one specific car and one specific person. Basically you look at your many-to-many association as a combinations of two one-to-many associations: I can now add custom data to the association and every association has its own primary key, so I can update the car or the owner reference in them.
Configure Many-to-Many Relationship in Code First
Note that this really is just a combination of two one-to-many relationships, so you can use all the configuration options discussed above. Mapping one-to-one The case when one person can be associated with one car and vice versa is called a one-to-one mapping. In this mapping, the relationship is always bidirectional: One-to-one mapping is probably the most interesting from a technical point of view.
There are a number of different scenarios worth discussing: One side can be optional and one can be required. Both sides can be required Both sides can be optional Mapping one-to-zero or one Let's discuss the case when one side is required and one side is optional. In our case it makes more sense for the car to be optional for the person. So let's modify the model a bit: The car has a required person HasRequiredwith the person having an optional car WithOptional.
From the Person side, it would look like this: Here's the actual script for the FK: They are in the database, but they are not foreign keys, as it might be expected. That's because one-to-one mappings does not support adding the FK into your EF model.