Managing One To Many Relationships With Entity Framework Core | Learn Entity Framework Core
This page covers how to add one-to-many entities using DBContext in Entity Framework Add One-to-Many Relationship Entity Graph using DBContext Entry and mark it as Added which result in same insert queries as above: dbCtx . Updating many to many relationships in Entity Framework Core. Last Updated: . Adding a Book many-to-many Author link. Creating new. Add a relationship (many-to-many); Add a relationship (FK-based); Update a related As with my ongoing project, the Entity Framework DbContext is . We use the same “Insert or Update' call – being aware that you have to.
Added Graphs in Disconnected Scenarios A common pattern that I use for handling reference lists is to use a separate context, which would no longer be in scope by the time you saved any user modifications.
This situation is common for Web applications and services, but it can also occur in a client-side app. The reason it happens is that when you use the DbSet. Add method that is, Screencasts.
Even though the developer may be aware that the Topic has an existing Id value, Entity Framework honors its EntityState Added and creates an Insert database command for the Topic, regardless of the existing Id. What if, instead of passing a graph to the repository, the repository method requests both the new Screencast and the selected Topic as parameters?
Instead of adding a full graph, it adds the Screencast entity and then sets its Topic navigation property: You might be familiar with using the EF Attach method to attach an untracked entity to a context. So, again, the Topic will be inserted into the database when SaveChanges is called. For example, if you were to attach the Topic directly to the context, instead of to the Added Screencast, its EntityState would start out as Unchanged. Setting it to screencast. Attach topicyou could set the state of the Topic before or after the fact, explicitly setting its state to Unchanged: Unchanged Calling this code before the context is aware of the Topic will cause the context to attach the Topic and then set the state.
Rather than setting the navigation property and having to worry about the state of the Topic, just set the TopicId property, because you do have access to that value in the Topic instance.
This is something I find myself frequently suggesting to developers. Even on Twitter, I see the question: SelectedValue ; Unfortunately, this brings you back to the original problem: EF sees the Topic entity in the graph and adds it to the context along with Screencast—even though the Screencast. All the unit tests and classes I use in this article can be found in the Chapter03 branch of the Git repo associated with this book.
If you are in a hurry I have added summaries for each section — just search for the word Summary to get the answers without needing to look at the code examples. Creating a many-to-many relationship in EF Core The example I am going to use is one taken from the book I am writing.
- The Problem Defined
- Creating relationships
- Entity Framework Core in Action book
It has a Book entity linked its Author s entities via a BookAuthor table. The database diagram below shows the three tables, with the foreign key links from the BookAuthor linking table to the other two tables.
A linking table like this has foreign keys FK in the diagram which link to the primary key s PK in the disgarm of each end of the relationship — in this case the BookId of the Book entity and the AuthorId of the Author entity. These foreign keys then form a composite primary key for the BookAuthor table.
Saving Related Data - EF Core | Microsoft Docs
Its not hard, so let me give you the code to define the BookAuthor entity class. I have a property called Order because the order of the authors matter. EF Core will find the relationships using its by convention rules because I have used names that it understands: Because I used the classes Book and Author, which EF Core knows are part of the database, then it knows its a relationship.
HasKey method towards the bottom of the code where I define the composite key consisting of the BookId and the AuthorId. But if you want to explicitly define the relationships you can, using the.
There are loads of configuration rules, which I cover in chapter 6 and 7 of my bookor you can look at the EF Core docs. You can set the keys, but not the relationships, via attributes. Summary — how to add a many-to-many relationship To add a many-to-many relationship in EF Core you need to: Create a linking entity class that has the foreign keys s of the two entities you want to form a many-to-many link between.
In my example these are BookId and AuthorId. It is efficient but not necessary to make these two foreign keys into the composite primary key.
This will ensure that there is only ever one link between the two entities you want. It is also more efficient in terms of size of the database table. You most likely want a collection relationship to the linking table in one or both of the linked entities. The code below creates a new book with a new author. Add book ; context.
Add One-to-Many Relationship Entity Graph using DBContext
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.