Top entity-framework Questions

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358
Chris Roberts

Now that .NET v3.5 SP1 has been released (along with VS2008 SP1), we now have access to the .NET entity framework.

My question is this. When trying to decide between using the Entity Framework and LINQ to SQL as an ORM, what's the difference?

The way I understand it, the Entity Framework (when used with LINQ to Entities) is a 'big brother' to LINQ to SQL? If this is the case - what advantages does it have? What can it do that LINQ to SQL can't do on its own?

Answered By: Kris ( 225)

LINQ to SQL only supports 1 to 1 mapping of database tables, views, sprocs and functions available in Microsoft SQL Server. It's a great API to use for quick data access construction to relatively well designed SQL Server databases. LINQ2SQL was first released with C# 3.0 and .Net Framework 3.5.

LINQ to Entities (ADO.Net Entity Framework) is an ORM (Object Relational Mapper) API which allows for a broad definition of object domain models and their relationships to many different ADO.Net data providers. As such, you can mix and match a number of different database vendors, application servers or protocols to design an aggregated mash-up of objects which are constructed from a variety of tables, sources, services, etc. ADO.Net Framework was released with the .Net Framework 3.5 SP1.

This is a good introductory article on MSDN: Introducing LINQ to Relational Data

209
J. Steen

All of a sudden I keep getting a MetadataException on instantiating my generated ObjectContext class. The connection string in App.Config looks correct - hasn't changed since last it worked - and I've tried regenerating a new model (edmx-file) from the underlying database with no change.

Anyone have any ideas?

Further details: I haven't changed any properties, I haven't changed the name of any output assemblies, I haven't tried to embed the EDMX in the assembly. I've merely waited 10 hours from leaving work until I got back. And then it wasn't working anymore.

I've tried recreating the EDMX. I've tried recreating the project. I've even tried recreating the database, from scratch. No luck, whatsoever.

Answered By: Craig Stuntz ( 234)

This means that the application is unable to load the EDMX. There are several things which can cause this.

  • You might have changed the MetadataArtifactProcessing property of the model to Copy to Output Directory.
  • The connection string could be wrong. I know you say you haven't changed it, but if you have changed other things (say, the name of an assembly), it could still be wrong.
  • You might be using a post-compile task to embed the EDMX in the assembly, which is no longer working for some reason.

In short, there is not really enough detail in your question to give an accurate answer, but hopefully these ideas should get you on the right track.

Update: I've written a blog post with more complete steps for troubleshooting.

What are the pros & cons of using Entity Framework 4.1 Code-first over Model/Database-first with EDMX diagram?

I'm trying to fully understand all the approaches to building data access layer using EF 4.1. I'm using Repository pattern and IoC.

I know I can use code-first approach: define my entities and context by hand and use ModelBuilder to fine-tune the schema.

I can also create an EDMX diagram and choose a code generation step that uses T4 templates to generate the same POCO classes.

In both cases I end up with POCO object which are ORM agnostic and context that derives from DbContext.

Database-first seems to be most appealing since I can design database in Enterprise Manager, quickly synch the model and fine-tune it using the designer.

So what is the difference between those two approaches? Is it just about the preference VS2010 vs Enterprise Manager?

Answered By: Ladislav Mrnka ( 217)

I think the differences are:

Code first

  • Very popular because hardcore programmers don't like any kind of designers and defining mapping in EDMX xml is too complex.
  • Full control over the code (no autogenerated code which is hard to modify).
  • General expectation is that you do not bother with DB. DB is just a storage with no logic. EF will handle creation and you don't want to know how it do the job.
  • Manual changes to database will be most probably lost because your code defines the database.

Database first

  • Very popular if you have DB designed by DBAs, developed separately or if you have existing DB.
  • You will let EF create entities for you and after modification of mapping you will generate POCO entities.
  • If you want additional features in POCO entities you must either T4 modify template or use partial classes.
  • Manual changes to the database are possible because the database defines your domain model. You can always update model from database (this feature works quite good).
  • I often use this together VS Database projects (only Premium and Ultimate version).

Model first

  • IMHO popular if you are designer fan (= you don't like writing code or SQL).
  • You will "draw" your model and let workflow to generate your database script and T4 template to generate yout POCO entities. You will lose part of control on both your entities and database but for small easy projects you will be very productive.
  • If you want additional features in POCO entities you must either T4 modify template or use partial classes.
  • Manual changes to database will be most probably lost because your model defines the database. This works better if you have Database generation power pack installed. It will allow you updating database schema (instead of recreating) or updating database projects in VS.

I expect that in case of EF 4.1 there are several other features related to Code First vs. Model/Database first. Fluent API used in Code first doesn't offer all features of EDMX. I expect that features like stored procedures mapping, query views, defining views etc. works when using Model/Database first and DbContext (I didn't try it yet) but they don't in Code first.

Can't find anything relevant about Entity Framework/MySQL on Google so I'm hoping someone knows about it.

Answered By: Vin ( 125)

It's been released - Get the MySQL connector for .Net v6.5 - this has support for [Entity Framework]

I was waiting for this the whole time, although the support is basic, works for most basic scenarios of db interaction. It also has basic Visual Studio integration.

I've been programming for a while and have used LINQ-To-SQL and LINQ-To-Entities before (although when using entities it has been on a Entity/Table 1-1 relationship - ie not much different than L2SQL)

I've been doing a lot of reading about Inversion of Control, Unit of Work, POCO and repository patterns and would like to use this methodology in my new applications.

Where I'm struggling is finding a clear, concise beginners guide for EF4 which doesn't assume knowledge of EF1.

The specific questions I need answered are:

Code first / model first? Pros/cons in regards to EF4 (ie what happens if I do code first, change the code at a later date and need to regenerate my DB model - Does the data get preserved and transformed or dropped?)

Assuming I'm going code-first (I'd like to see how EF4 converts that to a DB schema) how do I actually get started? Quite often I've seen articles with entity diagrams stating "So this is my entity model, now I'm going to ..." - Unfortunately, I'm unclear if they're created the model in the designer, saved it to generate code then stopped any further auto-code generation -or- They've coded (POCO)? classes and the somehow imported them into the deisgner view?

I suppose what I really need is an understanding of where the "magic" comes from and how to add it myself if I'm not just generating an EF model directly from a DB.

I'm aware the question is a little vague but I don't know what I don't know - So any input / correction / clarification appreciated.

Needless to say, I don't expect anyone to sit here and teach me EF - I'd just like some good tutorials/forums/blogs/etc. for complete entity newbies

Many thanks in advance

Answered By: KellySandwiches ( 42)

These articles might be of interest...the series really gets into the advantages and disadvantages of a POCO approach.

http://blogs.msdn.com/b/adonet/archive/2009/05/21/poco-in-the-entity-framework-part-1-the-experience.aspx

http://blogs.msdn.com/b/adonet/archive/2009/05/28/poco-in-the-entity-framework-part-2-complex-types-deferred-loading-and-explicit-loading.aspx

http://blogs.msdn.com/b/adonet/archive/2009/06/10/poco-in-the-entity-framework-part-3-change-tracking-with-poco.aspx

In these articles the author mentions future articles that describe best practices in implementing Repository and Unit of Work patterns, but I can't find them. These articles are well written and I'd like to read more from this author.

141
Keith Barrows

I am currently getting this error:

System.Data.SqlClient.SqlException: New transaction is not allowed because there are other threads running in the session.

while running this code:

public class ProductManager : IProductManager
{
    #region Declare Models
    private RivWorks.Model.Negotiation.RIV_Entities _dbRiv = RivWorks.Model.Stores.RivEntities(AppSettings.RivWorkEntities_connString);
    private RivWorks.Model.NegotiationAutos.RivFeedsEntities _dbFeed = RivWorks.Model.Stores.FeedEntities(AppSettings.FeedAutosEntities_connString);
    #endregion

    public IProduct GetProductById(Guid productId)
    {
        // Do a quick sync of the feeds...
        SyncFeeds();
        ...
        // get a product...
        ...
        return product;
    }

    private void SyncFeeds()
    {
        bool found = false;
        string feedSource = "AUTO";
        switch (feedSource) // companyFeedDetail.FeedSourceTable.ToUpper())
        {
            case "AUTO":
                var clientList = from a in _dbFeed.Client.Include("Auto") select a;
                foreach (RivWorks.Model.NegotiationAutos.Client client in clientList)
                {
                    var companyFeedDetailList = from a in _dbRiv.AutoNegotiationDetails where a.ClientID == client.ClientID select a;
                    foreach (RivWorks.Model.Negotiation.AutoNegotiationDetails companyFeedDetail in companyFeedDetailList)
                    {
                        if (companyFeedDetail.FeedSourceTable.ToUpper() == "AUTO")
                        {
                            var company = (from a in _dbRiv.Company.Include("Product") where a.CompanyId == companyFeedDetail.CompanyId select a).First();
                            foreach (RivWorks.Model.NegotiationAutos.Auto sourceProduct in client.Auto)
                            {
                                foreach (RivWorks.Model.Negotiation.Product targetProduct in company.Product)
                                {
                                    if (targetProduct.alternateProductID == sourceProduct.AutoID)
                                    {
                                        found = true;
                                        break;
                                    }
                                }
                                if (!found)
                                {
                                    var newProduct = new RivWorks.Model.Negotiation.Product();
                                    newProduct.alternateProductID = sourceProduct.AutoID;
                                    newProduct.isFromFeed = true;
                                    newProduct.isDeleted = false;
                                    newProduct.SKU = sourceProduct.StockNumber;
                                    company.Product.Add(newProduct);
                                }
                            }
                            _dbRiv.SaveChanges();  // ### THIS BREAKS ### //
                        }
                    }
                }
                break;
        }
    }
}

Model #1 - This model sits in a database on our Dev Server. Model #1

Model #2 - This model sits in a database on our Prod Server and is updated each day by automatic feeds. alt text

Note - The red circled items in Model #1 are the fields I use to "map" to Model #2. Please ignore the red circles in Model #2: that is from another question I had which is now answered.

Note: I still need to put in an isDeleted check so I can soft delete it from DB1 if it has gone out of our client's inventory.

All I want to do, with this particular code, is connect a company in DB1 with a client in DB2, get their product list from DB2 and INSERT it in DB1 if it is not already there. First time through should be a full pull of inventory. Each time it is run there after nothing should happen unless new inventory came in on the feed over night.

So the big question - how to I solve the transaction error I am getting? Do I need to drop and recreate my context each time through the loops (does not make sense to me)?

TIA

NOTE: Please look at the answer from Mark Stafford - MSFT below!

Answered By: Keith Barrows ( 155)

After much pulling out of hair I discovered that the foreach loops were the culprits. What needs to happen is to call EF but return it into an IList<T> of that target type then loop on the IList<T>.

Example:

IList<Client> clientList = from a in _dbFeed.Client.Include("Auto") select a;
foreach (RivWorks.Model.NegotiationAutos.Client client in clientList)
{
   var companyFeedDetailList = from a in _dbRiv.AutoNegotiationDetails where a.ClientID == client.ClientID select a;
    // ...
}