The best way to avoid this being an issue is to use a short-lived context for each unit-of-work such that the context starts empty, has entities attached to it, saves those entities, and then the context is disposed and discarded. You'll use separate queries for Enrollment entities and their related Student entities. If you include too much, you can end up with a lot of data that you dont need. Because the model that gets posted to the WebApi controller is detached from any entity-framework (EF) context, the only option is to load the object graph (parent including its children) from the database and compare which children have been added, deleted or updated. We have covered this in detail in one of our other articles: Creating a .NET Core Web API Controller. No need to concern yourself with that. Refresh the page again and select an instructor. When that happens, the HttpPost Delete method is called and then that method actually performs the delete operation. Is it possible for SQL Server to grant more memory to a query than is available to the instance. The following highlighted code shows the model validation check in the Create method. This will create the classes for supporting migrations. On the other hand, in some scenarios separate queries is more efficient. Is opposition to COVID-19 vaccines correlated with other political beliefs? Lets improve this: Better, but now we expose another problem. An entity must be tracked in the Added state to be inserted by SaveChanges. Get() Gets a specific employee record from the database by passing an Id. The Instructors page shows data from three different tables. This caching is often unnecessary in a web application because context instances are typically short-lived (a new one is created and disposed for each request) and the context that reads an entity is typically disposed before that entity is used again. When you call the Single method, you can also pass in the Where condition instead of calling the Where method separately: Next, if a course was selected, the selected course is retrieved from the list of courses in the view model. Remember there is one-to-one relationship between an aggregate and its related repository. We then use SetValues to set the values for all properties on this entity to those that came from the client. Starting with EF Core 3.0, calling DbContext.Entry will now only attempt to detect changes in the given entity and any tracked principal entities related to it. In more realistic examples, a graph of entities is first attached, and then some of those entities are marked as deleted. You can prevent overposting in edit scenarios by reading the entity from the database first and then calling TryUpdateModel, passing in an explicit allowed properties list. In this article. An alternative way to prevent overposting that's preferred by many developers is to use view models rather than entity classes with model binding. The purpose of this class is to separate the actual data operations logic from our API Controller. EF Core offers us a new place to add your update code to inside the entity class. However, this example shows how to use eager loading for navigation properties within entities that are themselves in navigation properties. EF Core automatically creates FKs in the database wherever they're needed. All the entities are marked as Unchanged, so there is nothing to update in the database. In theory, I could create a custom collection type that does all that, but Im not a fan of implicit database queries. For simplicity, this document uses and references synchronous methods such as SaveChanges rather than their async equivalents such as SaveChangesAsync. In the following Razor code, the id parameter matches the default route, so id is added to the route data. Find the latest U.S. news stories, photos, and videos on NBCNews.com. This code loops through the entities in the Enrollments navigation property. This typically results in a single join query that retrieves all of the data that's needed. By default, EF Core maps an inheritance hierarchy of .NET types to a single database table. But how can the DbContext know the state of children (new add/delete/modified)? Once we apply this command, we can see a SQL script generated with all changes related to our migrations. The parameters are provided by the Select hyperlinks on the page. You read an entity and make changes to some of its property values. Use _context.Entry on the entity instance to set its state to Unchanged, and then set Property("PropertyName").IsModified to true on each entity property that's included in the view model. This results in EF Core marking the new entity as Added instead of Unchanged: Calling SaveChanges at this point does nothing with the Unchanged entities, but inserts the new entity into the database. When the user selects an instructor, related Course entities are displayed. For example, when you pass a new entity to the Add method, that entity's state is set to Added. So each child in the dto will have a guid that may or may not be in the DB (update, add respectively). Each DbContext instance tracks changes made to entities. An alternative is to attach an entity created by the model binder to the EF context and mark it as modified. Other than the Bind attribute, the try-catch block is the only change you've made to the scaffolded code. The ASP.NET Core built-in dependency injection takes care of that task for you. Entity Framework Core will automatically fix-up navigation properties to any other entities that were previously loaded into the context instance. Is it really impossible to update child collection in EF out of the box (aka non-hacky way)? While working with the EF Core Code-First approach, we create the classes for our domain entities first. Well, no. Create a controller named CoursesController for the Course entity type, using the same options for the MVC Controller with views, using Entity Framework scaffolder that you did earlier for the StudentsController, as shown in the following illustration: Open CoursesController.cs and examine the Index method. For example: Internally, Add, Attach, and Update use graph-traversal with a determination made for each entity as to whether it should be marked as Added (to insert), Modified (to update), Unchanged (do nothing), or Deleted (to delete). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, See also example with GraphDiff in a duplicate question. In this situation, EF Core will, by default, delete dependent/child entities when the principal/parent is deleted. But calling Include starts over with Instructor properties, so you have to go through the chain again, this time specifying Course.Department instead of Course.Enrollments. I had this answer once but lost it along the way. So first, we create "OA.Data" project to implement this layer. Since EF tracking is ON, EF handles it like a charm. First, lets create a new Employee using a Post request: Next, lets do a Get request to get all Employees. By default, EF Core interprets a property that's named ID or classnameID as the primary key. Run the app and select the Instructors tab. We can just make the changes in our code and then sync everything easily with the database. The Single method throws an exception if the collection passed to it's empty or if there's more than one item. The extra round trips to the database are especially detrimental to performance when latency is high. Here are a few of the screens in the app: For information about other database providers that are available for EF Core, see Database providers. It handles all the mapping of properties so you don't have to do it manually. This allows the change tracker to automatically detect new entities and put them in the Added state. For example, to attach an existing blog and associated existing posts as Modified: The context is now tracking all these entities as Modified: Calling SaveChanges at this point will cause updates to be sent to the database for all these entities. However, sometimes entities are queried using one context instance and then saved using a different instance. So lets create some seed data now. When using the EF Core Code-First approach the best practice is to make all modifications to the database through the model and then update the database by doing the migration. It also drops AsNoTracking. By default the Entity Framework implicitly implements transactions. For information about repositories with EF, see the last tutorial in this series. We can see the new Employee record which was created in the previous request: Now, lets do a Put request to test the update functionality by changing the last name: Once again lets do a Get request and verify that the last name has changed: Now that we have successfully tested the API endpoints, lets verify that the changes we made are actually persisted in the database. Starting with EF Core 5.0, it can be created implicitly and hidden. Then when you call the SaveChanges method, the database context issues a SQL INSERT command. Read breaking headlines covering politics, economics, pop culture, and more. The CourseAssignments property contains CourseAssignment entities, from which you want only the related Course entities. See Additional Change Tracking Features for more information. The following code executes when an instructor was selected. 503), Fighting to balance identity and anonymity on the web(3) (Ep. Try entering an invalid date if your browser lets you do that. However, the last ORDER BY clause is not necessary for EF generate the needed groupings, and can have an impact in performance. Normally the entity instances will have come from another source, such as being deserialized from a client, or being created from data in an HTTP Post. Once we have set up the project, the next step is to set up the EF Core. 504), Mobile app infrastructure being decommissioned. The article is divided into the following sections: As a first step, lets set up an ASP.NET Core Web API Project. We will start by creating a folderModels within the root of the application. The new code reads the existing entity and calls TryUpdateModel to update fields in the retrieved entity based on user input in the posted form data. Updating related data with Entity Framework Core, Complete Insert/Update/Delete of Child Entities in Entity Framework. This entity's foreign key value must match the primary key value (or an alternate key value) of the related principal/parent entity. It's very simple. For example, to insert a new blog and associated new posts: The context is now tracking all these entities as Added: Notice that explicit values have been set for the Id key properties in the examples above. But there may be child items in the DB that are not in the list and should be deleted. For more information about tag helpers, see Tag Helpers in ASP.NET Core. It also provides a Select hyperlink that sends the ID of the selected course to the Index action method. Currently there are no extra fields that you're protecting, but listing the fields that you want the model binder to bind ensures that if you add fields to the data model in the future, they're automatically protected until you explicitly add them here. This method works in both edit and create scenarios. The method to get all posts is loading all the comments for every post even though theyre not needed. By default, integer and GUID key properties are configured to use automatically generated key values. The method that's called in response to a GET request displays a view that gives the user a chance to approve or cancel the delete operation. You are running a query that retrieves a large volume of data, and only a small portion of the returned data will be updated. Explicit loading: When the entity is first read, related data isn't retrieved. Added a new hyperlink labeled Select immediately before the other links in each row, which causes the selected instructor's ID to be sent to the Index method. As before, when not using auto-generated keys, a query and some processing can be used: Delete can be tricky to handle since often the absence of an entity means that it should be deleted. // We don't always load this, so it's nullable. We have explained this in detail in one of our other articles: Creating and configuring a new ASP.NET Core Web API project. Add-Migration EFCoreCodeFirstSample.Models.AddEmployeeGender. These values are used by EF Core until SaveChanges is called, at which point real key values are read back from the database. In that case, you might want to load the enrollment data only if it's requested. Modified. Now lets verify that the database and tables are created by opening SQL Server Management Studio or Visual Studio Server Explorer: We can see the database EmployeeDBis created with a table Employeeswhich contains the columns based on the fields we defined in our model. This can be done in the ConfigureServicesmethod intheStartup.csas below: Now that our DataManager is all set, lets create the API Controller and create the endpoints for handling CRUD operations. This means that changes elsewhere may not have been detected by calling this method, which could have implications on application state. When SaveChanges is called, a SQL DELETE command is generated. It is unusual to call Remove on an entity created with new. Entity Framework Core 1.0 doesn't support lazy loading. But I agree that hidden database queries like that should be avoided. As a best practice to prevent overposting, the fields that you want to be updateable by the Edit page are declared in the TryUpdateModel parameters.