Configuring One To Many Relationships in Entity Framework Core | Learn Entity Framework Core
One-to-One Relationships; One-to-Many or Many to One Relationships We will create two class library projects, one library project (EF. . The constructor of the UserMap class uses the Fluent API to map and configure. Here, we will learn how to configure One-to-Many relationships between two entities (domain classes) in Entity Framework 6.x using the code-first approach. Entity Framework Code First relationship mapping (One-to-many, Many-to-many, One or zero-to-One).
The Property method configures attributes for each property belonging to an entity or complex type.
It is used to obtain a configuration object for a given property. The options on the configuration object are specific to the type being configured. It configures how values for the property are generated by the database. DatabaseGeneratedOption is the database annotation. It enumerates a database generated option.#12 Hibernate Tutorial - Mapping Relations Theory
Identity is used to create an auto-increment column in the table by a unique value. Configures the table name that this entity type is mapped to.
Configures a required relationship from this entity type. Instances of the entity type will not be able to be saved to the database unless this relationship is specified.
The foreign key in the database will be non-nullable. The entity type being configured will be the dependent and contain a foreign key to the principal. The entity type that the relationship targets will be the principal in the relationship. Now define the connection string in App.
Data project so that we can create database with the appropriate name. In this class, we override the OnModelCreating method. This method is called when the model for a context class EFDbContext has been initialized, but before the model has been locked down and used to initialize the context such that the model can be further configured before it is locked down.
The following is the code snippet for the context class. CreateInstance type ; modelBuilder. Config file and it connects to that server. 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.
Configuring One To Many Relationships in Entity Framework Core
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. 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.
Relationship in Entity Framework Using Code First Approach With Fluent API - CodeProject
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. Let's look at an example of all the conventions which create a one-to-many relationship.
Convention 1 We want to establish a one-to-many relationship between the Student and Grade entities where many students are associated with one Grade. It means that each Student entity points to a Grade. This can be achieved by including a reference navigation property of type Grade in the Student entity class, as shown below. So, there can be many students in a single grade. Convention 2 Another convention is to include a collection navigation property in the principal entity as shown below. This also results in a one-to-many relationship between the Student and Grade entities.
Entity Framework Code First: Relationship mapping
This example produces the same result in the database as convention 1. Convention 3 Including navigation properties at both ends will also result in a one-to-many relationship, as shown below. Convention 4 A fully defined relationship at both ends will create a one-to-many relationship, as shown below. This will create a one-to-many relationship with the NotNull foreign key column in the Students table, as shown below.