(Because this is a one-to-zero-or-one relationship, there might not be a related OfficeAssignment entity.). If the key is set and the value is non-negative, then the entity is marked as Modified. Following the article linked above, lets create a new project calledEFCoreCodeFirstSample. explicitly set) key values are shown first in each example because everything is very explicit and easy to follow. Creating new entities that will be inserted into the database. Lets run the migration commands once again: Add-Migration EFCoreCodeFirstSample.Models.EmployeeContextSeed. (Don't update your project with this code, it's only shown to illustrate an optional approach.). The new code drops the ThenInclude method calls for enrollment data from the code that retrieves instructor entities. Deletes then becomes the same as updates. The key value should only be used in the way shown here before any call is made to track the entity. Enter names and a date. For example, to start tracking an existing blog: The examples here are creating entities explicitly with new for simplicity. Why are standard frequentist hypotheses so uninteresting? This code adds the Student entity created by the ASP.NET Core MVC model binder to the Students entity set and then saves the changes to the database. A disconnected entity, perhaps returned from a web client in an HTTP request, can be put into this state using either DbContext.Attach, DbContext.AttachRange, or the equivalent methods on DbSet. Soft deletes can be implemented using query filters. Notice the AutoMapper <3. If an entity with the same keys is already present EF will include it in the results of the query. In the EF Core Code-First approach, we have full control over the code and the database is just a store without any logic. The debug view is therefore empty because no entities are being tracked. In this case, separate queries might be more efficient because you need courses only for the selected instructor. The page displays the Location property of related OfficeAssignment entities and an empty table cell when there's no related OfficeAssignment entity. You'll use eager loading for the OfficeAssignment entities. With this approach to entity deletion, EF might not realize there are related entities to be deleted. But why ef doesn't have a more "brilliant" way? How do I stop Entity Framework from trying to save/insert child objects? To learn more, see our tips on writing great answers. Why? When not using generated keys, the key properties must be explicitly set before calling Add. What if you're using client-generated guids and the entity is in a disconnected/detached state? If the user approves it, a POST request is created. This code retrieves the selected entity, then calls the Remove method to set the entity's status to Deleted. Would call the repo instead of direct calls to DbContext.Entry. Attaching entities to the same DbContext instance that they were queried from should not normally be needed. You've been Haacked is a blog about Technology, Software, Management, and Open Source. Gets all employee records from the database. The value "OverPost" would then be successfully added to the Secret property of the inserted row, although you never intended that the web page be able to set that property. This project holds POCO class and fluent API configuration for this POCO classes. See Additional Change Tracking Features for more information. In Startup.cs, you call the AddDbContext extension method to provision the DbContext class in the ASP.NET Core DI container. For an entity to be deleted by SaveChanges it must be tracked in the Deleted state. Maybe that's not a problem for you but it's more expensive and I want to log exact changes inside Save so I need correct info. Heres one approach that you might take (with some properties omitted for brevity): In our hypothetical web app, every page that displays a blog post will need to display the posts authors, but only some of them will display comments. The update action looks like: Get a tracked parent entity named existing by model.Id, and assign values in model one by one to the entity. Create the data model. Include only the properties you want to update in the view model. Good luck! It's not the most elegant procedure, but it works. Please keep it to C# answers for C# questions :), if you are used to search for VB answer, usually you will end up finding the answers in C#, --!--, How to add/update child entities when updating a parent entity in EF, entityframework.codeplex.com/workitem/864, Going from engineer to entrepreneur takes more than just good code (Ep. As a result of these changes, the method signature of the HttpPost Edit method is the same as the HttpGet Edit method; therefore you've renamed the method EditPost. 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. Creates a new employee record in the database. For example, to start tracking an existing blog as Modified: Inspecting the change tracker debug view following this call shows that the context is tracking this entity in the Modified state: Just like with Add and Attach, Update actually marks an entire graph of related entities as Modified. As a next step, lets configure the repository using dependency injection. For example, to insert a new blog and posts all with generated key values: As with explicit key values, the context is now tracking all these entities as Added: Notice in this case that temporary key values have been generated for each entity. Id rather be explicit about when my code calls the database. For example, deleting a blog with related posts as in the previous example: Inspecting the change tracker debug view following the call to Remove shows that, as expected, the blog is again marked as Deleted: More interestingly in this case is that all related posts have also been marked as Deleted. Suppose, for some reason, earlier in the same request with the same DbContext we load a comment like this: And this comment belongs to the same post that were trying to get the comment count for. When working with graphs the graph should ideally be created such that this invariant is maintained, and the context should be used for only one unit-of-work. The database context keeps track of whether entities in memory are in sync with their corresponding rows in the database, and this information determines what happens when you call the SaveChanges method. 3.1.1 Getting entities from the object cache using DbContext Find If you include too much, you can end up with a lot of data that you dont need. Calling SaveChanges updates the foreign key value for each post to null in the database, before then deleting the blog: After SaveChanges completes, the deleted entity is detached from the DbContext since it no longer exists in the database. For example, often the request to insert a new entity is different from the request to update an existing entity. The AsNoTracking method improves performance in scenarios where the entities returned won't be updated in the current context's lifetime. Next, lets create a concrete class that implements the interfaceIDataRepository. I left the null check as an optimization, but its not strictly necessary. Try it in EF6 | Try it in EF Core. // We don't always load this, so it's nullable. Replace the HttpPost Edit action method with the following code. Refresh the page and select an instructor. Likewise, if all the entities in a graph need to be updated, then Update can be used: The blog and all its posts will be marked to be updated. More info about Internet Explorer and Microsoft Edge, ChangeTracker.TrackGraph(Object, TState, Func,Boolean>). You can replace the whole list with a new one! If the graph does contain duplicates, then it will be necessary to process the graph before sending it to EF to consolidate multiple instances into one. But if you don't want to do the extra read operation, you have to use the entity object created by the model binder. I hope you find it useful. If you want, you have a place to learn a lot more about this topic. Course entities are required when an instructor is selected in the web page, so a single query is better than multiple queries only if the page is displayed more often with a course selected than without. Contains all of the Enrollment entities that are related to that Student entity. More info about Internet Explorer and Microsoft Edge. absolute torture when you know there's a better way but can't remember it or find it! Asking for help, clarification, or responding to other answers. DbUpdateException exceptions are sometimes caused by something external to the application rather than a programming error, so the user is advised to try again. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. This is because the model here has been configured to use explicitly set key values, rather than automatically generated key values. The second is only needed by applications that change entities or their relationships while the entities are not being tracked. EF Core lets you write code to execute CRUD actions (create, read, update, and delete) without understanding how the data is persisted in the underlying database. We want to modify our collection in a way that EF tracking won't end up messed up. Why? An alternative way to prevent overposting that's preferred by many developers is to use view models rather than entity classes with model binding. We have explained this in detail in one of our other articles: Creating and configuring a new ASP.NET Core Web API project. If an exception that derives from DbUpdateException is caught while the changes are being saved, a generic error message is displayed. 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. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Each DbContext instance tracks changes made to entities. All the information and the code come from Chapter 2 of the second edition of my book, Entity Framework Core in Action, which cover EF Core 5. However, the last ORDER BY clause is not necessary for EF generate the needed groupings, and can have an impact in performance. Is a potential juror protected for what they say during jury selection? Updating child objects in Entity Framework 6, EF how to update entity containing the List of entities, Entity Framework: CurrentValues cannot be used for entities in the Deleted state. Entity Framework Core (EF Core) change tracking works best when the same DbContext instance is used to both query for entities and update them by calling SaveChanges. In that case, you might want to load the enrollment data only if it's requested. Unfortunately, this kinda falls over if there are collection properties on the child type which also need to be updated. The scaffolder generated a Bind attribute and added the entity created by the model binder to the entity set with a Modified flag. Directly accessing the context methods from the API controller is a bad practice and we should avoid that. There are several ways that Object-Relational Mapping (ORM) software such as Entity Framework can load related data into the navigation properties of an entity: Eager loading: When the entity is read, related data is retrieved along with it. On the other hand, if the key value has been set, then it must have already been previously saved and now needs updating. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Just proof of concept Controler.UpdateModel won't work correctly. 2022. You don't need to change this method. It turns out that even though we havent explicitely included or loaded post.Comments, it will be a non-null collection with one entry, the comment. Remember there is one-to-one relationship between an aggregate and its related repository. This wasnt really possible in EF6.x. Add code that calls Include. That is, EF automatically adds the separately retrieved entities where they belong in navigation properties of previously retrieved entities. Scoped means the context object lifetime coincides with the web request life time, and the Dispose method will be called automatically at the end of the web request. Advance to the next tutorial to learn how to expand the functionality of the Index page by adding sorting, filtering, and paging. Find centralized, trusted content and collaborate around the technologies you use most. Is there any lib that adds/updates child entities when updating a parent entity in Entity Framework Core, without write a lot of manual code? rev2022.11.7.43014. For more information, see Tracking vs. No-Tracking. Hope you enjoyed the article. For simplicity, this document uses and references synchronous methods such as SaveChanges rather than their async equivalents such as SaveChangesAsync. Ive never used EF so Im not familiar with it. Each field is displayed using DisplayNameFor and DisplayFor helpers, as shown in the following example: After the last field and immediately before the closing tag, add the following code to display a list of enrollments: If code indentation is wrong after you paste the code, press CTRL-K-D to correct it. If Find returns null, then the database doesn't already contain the blog with this ID, so we call Add mark it for insertion. The process is explained in detail in one of our other articles: Context Class and the Database Connection, Following the above article, lets define the context file, and lets define the database connection in the. An entity with a foreign key is the child or dependent entity in the relationship. The course title is retrieved from the Course entity that's stored in the Course navigation property of the Enrollments entity. I dont undestand why you set Children again while you Include it in the initial query? Don't be too scared by this; for many applications with small graphs, this can be an easy and pragmatic way of generating updates. If the entity is not using auto-generated keys, then the application must decide whether the entity should be inserted or updated: For example: SetValues will only mark as modified the properties that have different values to those in the tracked entity. Frequently these conditions are met in a controller's HttpGet action methods. The one and only resource you'll ever need to learn APIs: Want to kick start your web development in C#? This indicates that the dependents/children are no longer related to any principal/parent. Is it really impossible to update child collection in EF out of the box (aka non-hacky way)? Then lets create a new interface calledIDataRepository: We will later inject this interface into our API Controller and API will be communicating with the data context using this interface. This typically results in a single join query that retrieves all of the data that's needed. 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. Eager loading of all related data in one query might cause a very complex join to be generated, which SQL Server can't process efficiently. Then when you call SaveChanges, the Entity Framework updates all columns of the database row, because the context has no way to know which properties you changed. This can lead to unexpected behavior. You see the list of courses and grades for the selected student: In StudentsController.cs, modify the HttpPost Create method by adding a try-catch block and removing ID from the Bind attribute. 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. By default the Entity Framework implicitly implements transactions. Then lets install the Microsoft.EntityFrameworkCore.SqlServer package and register our context in the Startup.cs: Our next step is to add Code-First Migrations. EF Core has a feature called backing fields that makes building DDD entities possible. This is primarily useful when: The first of these will be needed by most applications, and is primarily handled by the DbContext.Add methods. amList: is the child list that you want to add or modify, rList: is the child list that you want to remove. These methods all work in the same way in the context of change tracking. This will create the classes for supporting migrations. You are running a query that retrieves a large volume of data, and only a small portion of the returned data will be updated. This causes its entity state to automatically be changed to Modified. For example, to start tracking a new blog: Inspecting the change tracker debug view following this call shows that the context is tracking the new entity in the Added state: However, the Add methods don't just work on an individual entity. For scenarios where you need more control -- for example, if you want to include operations done outside of Entity Framework in a transaction -- see Transactions. So first, we create "OA.Data" project to implement this layer. For example, if the entity is returned from a query that includes the collection, then the collection is non-nullable and fully loaded. This often happens in "disconnected" scenarios such as a web application where the entities are queried, sent to the client, modified, sent back to the server in a request, and then saved. In the simple overload used above, EF Core determines when to stop traversing the graph. For example: All entities are marked as Unchanged, except the one on which Remove was called: After SaveChanges completes, the deleted entity is detached from the DbContext since it no longer exists in the database. For true deletes, a common pattern is to use an extension of the query pattern to perform what is essentially a graph diff. Automatic fix-up does not set IsLoaded to true for the collection. 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. Changed the Department column to display the department name. So we should make changes to the code only. The Single method throws an exception if the collection passed to it's empty or if there's more than one item. Happy programming! In Entity Framework Core 1.1 you can use the Load method to do explicit loading. So even if you dont explicitly include the data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded. One solution is to make the collection nullable. Following are the steps for configuring the EF Core: Defining the Model; Creating a Context File; Generating the Database from Code Using Migrations; Defining the Model. This generates the following HTML when item.ID is 6: In the following Razor code, studentID doesn't match a parameter in the default route, so it's added as a query string. The source code for this blog post can be found on this Github repo. In this article, we have learned the following topics. Run the app and select the Instructors tab. This means that when the update is sent, only those columns that have actually changed will be updated. This approach is helpful in situations where we are starting with the development of a new project and we dont have a clear picture of what our database should look like yet. The Unchanged state means that the entity has not been modified since it was queried. For simplicity, this code assumes each entity has an integer primary key property called Id. What is the proper way to update child entities in EF 6? One way to deal with this is to use "soft deletes" such that the entity is marked as deleted rather than actually being deleted. One approach we could take is to always load both collections: This is a simple approach, but not really scalable. Nothing needs to be done with this entity by the SaveChanges method. The selected instructor is retrieved from the list of instructors in the view model. Ideally. Heres how I ended up fixing it: The call to _dbContext.Entry(post).Collection(p => p.Comments).IsLoaded tells us if the collection is fully loaded or not. So even if you dont explicitly include the data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded. I know its boring lazy to use the example of a blog post to illustrate this concept in a blog post, but its a well understood domain and its whats often used in EF Cores own documentation. Suppose you expected users to only rarely want to see enrollments in a selected instructor and course. The view model's Courses property is then loaded with the Course entities from that instructor's CourseAssignments navigation property. For more information about tag helpers, see Tag Helpers in ASP.NET Core. Entities can be explicitly "attached" to a DbContext such that the context then tracks those entities. The difference is that with explicit loading, the code specifies the navigation properties to be loaded. Change the date to a valid value and click Create to see the new student appear in the Index page. 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. won't work for partial update of an entity). As noted above, EF Core can only track one instance of any entity with a given primary key value. Eager loading of all related data would result in just a single (join) query and a single round trip to the database. In this section, you'll create a controller and view for the Instructor entity in order to display the Instructors page: This page reads and displays related data in the following ways: The list of instructors displays related data from the OfficeAssignment entity. However, in this case the primary key is meaningful and you want to show it. The Instructor and OfficeAssignment entities are in a one-to-zero-or-one relationship. As you saw for update and create operations, delete operations require two action methods. Configuring EF Core. The process is explained in detail in one of our other articles: Context Class and the Database Connection. Many-to-many relationships in EF Core are implemented using a join entity. This code loops through the entities in the Enrollments navigation property. Target Framework. @RalphWillgoss What's the fix in 2.2 you were talking about? Not doing so would leave a foreign key value referencing a primary key value that no longer exists. When you read an entity from the database, the entity starts out with this status. 504), Mobile app infrastructure being decommissioned. Then you can create a Student entity using the original values, call the Attach method with that original version of the entity, update the entity's values to the new values, and then call SaveChanges. We can use the same command by specifying the name of the previous migration: update-database EFCoreCodeFirstSample.Models.EmployeeContextSeed. To see an example of how to do explicit loading, replace the Index method with the following code, which removes eager loading for Enrollments and loads that property explicitly. For example, to mark an existing post as Deleted: Inspecting the change tracker debug view following this call shows that the context is tracking the entity in the Deleted state: This entity will be deleted when SaveChanges is called. However, if a negative key value is found, then its real, non-negative value is restored and the entity is tracked as Deleted. To bring it to sync with the model, lets add another migration. When you retrieved the list of instructors in InstructorsController.cs, you specified eager loading for the CourseAssignments navigation property.
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