I'm currently creating an application which is highly modular (using the Prism-Framework) and accesses a database via the EntityFramework implemented in CodeFirst.
My goal is to seperate the actual writing of the data into the Database from the "normal" use of the created Entities. Writing to the Database shall only be done by the main-Application but the Modules should still be able to use the Entity-Classes.
Thus, they must know the DataContext or at least the Entiy Classes. Here is the problem, though: If a module changes a property of an Entity Class and the main-Application calls "SaveChanges()" on the DataContext for some other reason, the changes made by the module are automatically saved to the Database without the main-Application having control over it.
How can I prevent this behaviour? The Modules must not be able to change the Database-Content, except via a defined Interface to the main-Application.
My first thought was to implement ICloneable in every entity-Class and to only pass clones of the Entity-Objects to the Modules to work with. The modules would then, if they wanted to request a change in the database, pass the cloned Objects to the main-Application which updates the original object and calls "SaveChanges()" on the DataContext.
Do you guys think this is a viable solution, or might there be a better way to implement this behaviour?
Thanks in advance!
Use the DbSet.AsNoTracking() Method to enable reading data from the database that will not be tracked by the DbContext.
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We are currently rewriting an existing internal ASP.NET Web Forms application. Our application consists of a Web Api back end which uses Entity Framework 6 for data access and an front end which uses AngularJS.
We have an existing large database that I've created EF models using the Code-First Using Existing Database method and we are using data transfer object classes as inputs/outputs to our API methods so we aren't directly exposing our model classes. So basically, I'm trying to become proficient with EF, Web Api and AngularJS all at the same time. For the most part I'm fairly comfortable with the latter two, but for EF I haven't completely gotten comfortable with. I've watched a lot of the videos on Microsoft Virtual Academy but this is the first time I've had some hands-on experience with it.
We've been working on this application for a few months and so far we've only had to work with CRUD operations on our entities (POCO DTO's) which are flat objects with simple properties. However, we've finally come across some situations where we need to deal not only with our classes, but properties which are classes themselves; a parent-child relationship.
Therefore, I have the following questions:
I see that when we have a proper foreign key relationship in our DB, that virtual properties are created in EF, which from what I recall are to support lazy-loading. However, lazy-loading isn't really feasible in this environment where we are using web services (Web Api). Our object model does allow for some really large hierarchy of classes where a fully populated object and its children would mean a large amount of data would be passed around when that really isn't necessary, so in most cases a first level object is all we need. In some cases however, we do want to populate child classes, so my question is how do we do that, and where do we do that? I've looked at the automatically-generated code in the DB Context but we have also used scaffolded code to create our controllers. Which place do we need to do this? I've seen code samples showing how do to this but it hasn't said specifically where this code lies. It appears to be within a controller but I could be wrong.
If we do allow for 2- or more level hierarchy of objects, does EF automatically handle operations (updates, deletes, etc.) -- for example, if we have a "Company" object which has a collection of "Customer" objects, and we delete the "Company" object, do the related "Customer" objects get deleted too? Also, is a multi-step operation like that automatically performed within a transaction or do we need to explicitly set that up?
If I modify a model class or the DB context, seeing as this code is automatically-generated, that's generally bad practice as my changes could be overwritten, so I am assuming the controller code is where I want to make my changes. I am aware of database migrations but I have no experience with them and I am sure I'll need to use them at some point because I am fairly confident that our database may not have all the foreign key relationships necessary for EF to do everything we need at the moment.
I know this is a long post, but if anyone can give some guidance on how to do some of these things because it's not only me that's having to deal with this but I have two other developers on my team who are working on this project and we are all as inexperienced with this as the others are. Thanks
For the purpose of sending data across a web service, I'd suggest creating a DTO to hold the data you want to send and mapping your entities to the DTO instead of trying to send the entities themselves in your payload. It also protects your API from changes to your entity.
Cascading deletes are configurable, iirc, but I'm not 100% sure what the default is. Transactions are generally not implicit, so you will want to use those where you require them.
Not exactly sure what you are asking here. In general, how your entities/tables change depends on if you are using database-first or code-first. If you are using database-first (you will have a .edmx file in your solution that has the model matching your schema), you just update the SQL directly and update your entity model via the .edmx. If you use code-first, you will change the entities how you want them and run a database migration to update your database to match.
MSDN article about code-first migration: https://msdn.microsoft.com/en-us/data/jj591621.aspx
What I'd LIKE to do is manipulate EF to support plugins that access a shared database. So the database would contain all of the tables for the main application plus all of the tables required for each plugin. As the application doesn't know anything about the plugin data structures, it cannot be responsible for their management. The plugins are solely responsible, and create the tables themselves. However, the plugins know about the host application and its data structures, so ideally should be able to reference them and even inherit from them, resulting in a database that is extensible yet able to implement optimized patterns.
In EF, this translates to a HostContext that contains the DbSets appropriate for the Host. Each Plugin, I thought, should have a PluginContext that inherits from HostContext that contains the DbSets needed by the plugin. The entity classes included in PluginContext would then be able to reference HostContext entities, and/or inherit from those entities, and EF would be able to resolve the table mapping and relationships.
I'm using EF6. When I attempt the above and try to list the single entity I've included in the PluginContext, an exception is thrown complaining that the entity doesn't exist. Sure enough, no matching table has been created.
Is what I'm attempting to do supported by EF? If so, what am I doing wrong?
As I mentioned here: EF6 Empty Model target string
I began this effort using Rowan Miller's example here: http://romiller.com/2013/02/15/extending-and-customizing-code-first-models-part-2-of-2/
In the end, I abandoned that approach, for a few reasons: 1) I couldn't get it to work (I can't remember exactly why but I do suspect it was related to differences in EF since the article was written), and 2) I didn't like the need to write manual migrations.
I ended up with PluginContexts that inherit from HostContext, as I had hoped, and am able to reference and even inherit from host entities. This has restrictions in its use though:
My plugin logic is completely self contained. I have no need for the host application to manipulate or create plugin entities. Therefore, I am not trying to get the system to subsitute any plugin entities for host entities. If construction of a particular entity subclass is required, then a plugin method must be provided for that and an inheritence hiearchy will be utilized.
Migrations can be built even on the plugin context as per normal. However, that migration may easily include migration code from the Host Context. So I have to remember to look for and remove these instructions. This is typically very straightforward and I believe is much less effort than building the equivalent from scratch.
If there is any change to the Host Context then this must be reflected in every Plugin Context. Basically, this means anytime a new migration is created to reflect changes in Host Context, migrations must be created for each plugin as well, even though that migration may be empty (it isn't really - the critical part here is updating the Model in the latest MigrationHistory record to reflect the Plugin model that has been changed because of the inherited Host model).
This approach is being used to extend an in-house application with in-house plugins, and so may not be as easy to adopt in other scenarios which Rowan's solution is probably better suited.
I am playing around with Entity Framework to see how it can be used in a new project I am working on. I put my edmx file in a class library so the Entities (and database access) can be used in multiple places. Currently I have a web project and a console project both referencing the class library.
One of my Entities has a Partial class defined with a static method. The purpose of the method is to accept some parameters and create one or more instances of the specific class. My first version of the method created an ObjectContext instance, created the Entity class (or classes), and returned the Entities to the calling method. The calling method then updated some properties and tried to save the Entities using a new ObjectContext instance. Obviously this did not work because the Entities were bound (correct term ??) to the Context created within the static method.
After some research, I modified the static method to also accept an ObjectContext reference to ensure that all the Entities where created and then later on manipulated and saved using the same Context. This works fine but the design just feels wrong.
Assuming that my one static method may grow into many more, or that my app (especially the web app) would probably benefit from additional layers (DAL or even a Service Layer), does it make sense for all these classes to require an ObjectContext parameter?
I have read on many postings that creating an ObjectContext via a Singleton pattern is a bad idea because "many clients would use the same object". My problem with that is I do not see how that is possible. In a local console app there is only a single user running the app. In a web app there would only be a single user on each request. Where is the user sharing problem? Not a single article/posting mentioned it...but where they referring to a Singleton pattern storing the object instance in the Application context?
I have also seen postings focused on web usage and storing the object instance in the users Session object via the HttpContext object. This makes sense but does not seem to address non-web usage.
I think that whatever solution is appropriate (static methodm, Factory object, etc.) would most likely be implemented in my class library so obviously it needs to support both web and non-web solutions. Maybe check for HttpContext to determine what environment it is running in.
I was hoping http://www.west-wind.com/weblog/posts/2008/Feb/05/Linq-to-SQL-DataContext-Lifetime-Management would be informative but I am having a hard time wrapping my head around the post and the example code seems like overkill for instantiating and sharing a simple object. (Although I am sure I am just not getting it...)
Any thoughts are appreciated.
Thanks.
The issue is not that “many clients would use the same object.” The issue is that the ObjectContext is intended to be a single unit of work. If you use it for many different units of work, you will find that there are a number of problems.
Memory usage will grow and grow.
Your application will become slower as object fixup has to do increasing amounts of work.
Multithreading won't work
The solution is to use the ObjectContext in the manner it is intended, namely, as a single unit of work.
Can anyone direct me to a good strategy for implementing change tracking in my Entity Framework model?
I have around 20 entities to track changes on (accessed via facades / unit of work) and I need to be able to display who changed what when on displaying the record in the UI.
I know there's Context.OnSavingChanges (or whatever it's called) but I'd probably want to access the changes in queries like context.MyEntity.ChangeLog
Must I create a ChangeLog entity, add associations to all the entities or is there a better via via savingchanges?
Richard
P.s. Have a great weekend!
Entity framework is ORM = API responsible for persistence and loading from database. What you persist or load is completely up to you so if you want change tracking you must to code it.
The most common approach is indeed using OnSavingChanges or overriding SaveChanges because you are usually saving changed executed by single user.
An old question but for anyone looking for auditing changes on EF >= 6 or EF Core, I worked on an open source library Audit.EntityFramework you could try.
See FrameLog, which is an open source library I wrote for this purpose. You call it from SaveChanges and it deals with the rest, including giving you a strongly-typed API for querying the logs.
I'm developing a program which allows users to input some information which then gets stored and dynamically creates an image based on it.
I was going to use the Entity Framework to do the work with the data, but then I obviously need a way to generate the image. My thinking was that the "correct" way to do this was to somehow extend the data entity to include a function call like "CreateImage", or alternatively, to create a separate class not in the EF called "DataImage" which would have a "generate" method.
Extending the EF seems the "pure" way to do this, but I'm not sure how or if it's more practical than using the separate class.
Any thoughts on the best way to do this and how to do it using EF?
Putting this functionality in the EF would be a major violation of SRP. Breaking SRP has cascading negative effects as your application grows.
The approach you most likely want to take instead is a totally separate, encapsulated image generation service which takes interfaces that your EF entities implement. This decouples your image service from your data access completely; you get complete testability and zero dependencies right away.