Entity Framework Code First: Relationship mapping
When working with Code First, you define your model by defining your By default, Entity Framework uses the Code First conventions to map your classes to the The following code configures a many-to-many relationship. shows how to configure many-to-many relationship between entities using code first The Map() method takes Action type delegate, hence, we can pass the. Mapping many-to-many relationships with a join entity/table; Abstracting/hiding the join The join entity is not gone; it's still in application code and still mapped. A good example of a many-to-many relationship is blogs and tags. . First, we need a DbContext: . Part 2: The codeIn "Entity Framework".
And that's because one-to-one mappings are quite problematic in a relational database. The idea is that every person can have exactly one car, and that car can only belong to that person. Or there might be person records, which do not have cars associated with them. So how could this be represented with foreign keys? 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. 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!
Fluent API - Relationships - EF6 | Microsoft Docs
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'.
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.
Configure One-to-Many Relationships in EF 6
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: Add book ; context.
SaveChanges ; Updating a many-to-many relationship It turns out there are two scenarios under which you want to update a relationship. You find this sort of update that happens in a console application, or inside your business logic see this link on business logic and EF Core Disconnected state: This is where the update is split into two halves: This happens on web sites, where in the first stage the user picks what they want done and posts it back. The second stage then has to re-read the data and update it.
I am going to describe these two approaches separately, so you can go to the one that fits the application you are building. Connected state update In the connected state we read in the Book entity and update it immediately, i. I am using a Sqlite, in-memory database, which I seed with four books with known titles and authors. Here I load the Book entity using an Include method to load the AuthorLinks at the same time, as tracked entities. The default way of loading data in EF Core is as tracked entities, that is, EF Core takes a copy of the loaded data so that it can detect if any changes have happened since they were loaded.
You can turn off tracking by including the method. AsNoTracking to your query useful in read-only queries, as it makes them a little faster.
Summary — Connected state many-to-many update To update a many-to-many relationship in the connected state: In my example I loaded the Book entity I wanted to change along with its AuthorLinks property, which holds the collection of the linking entities in the BookAuthor table. Alter the linking table collection, either by adding or removing entries from the collection. Disconnected state The disconnected state happens when the initial read of the data and update of the data are done separately, i.Part 23 Many to many relationship in entity framework code first
This happens in a web application which has two stages: The first stage is where the user is presented with the book, its current author s and a list of possible authors to choose from. Once they have chosen the author to add then press a button which takes their choice back to the web application In the second stage the web application needs to re-read the data and execute the database update.
This unit test uses two, separate instances of the DbContext to simulate the disconnected state. With the help of Daria see comment below I have found a way to use Sqlite in-memory with multiple instances of the DbContext.
Configure One-to-Many Relationship in Entity Framework 6
Summary — Disconnected state many-to-many update To update a many-to-many relationship in the disconnected state: The one-to-many relationship can be configured in the following ways. By using Fluent API Configurations Conventions for One-to-Many Relationships There are certain conventions in Entity Framework which if followed in entity classes domain classes will automatically result in a one-to-many relationship between two tables in the database.
You don't need to configure anything else. 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. This example produces the same result in the database as convention 1.