Im using visual studio, postgresql database and ado.net entity data model. In the connectionstring, Im unable set MultipleActiveResultSets=True.
Usually when I connect to sql server with MultipleActiveResultSets=True, it works fine. but i cannot set the same with postgresql database.
When I use this, I got the following error
There is already an open DataReader associated with this Command which
must be closed first.
How do I solve this problem.
Multiple Active Result Sets (MARS) is a feature introduced in SQL Server 2005 and is not available in other database systems like postgres so you won't be able to turn it on in the connection string.
The error you are facing is an outcome of trying to perform two queries on one open data reader. When using ie Entity Framework this usually happens when you have Lazy Loading turned on and the lazy properties are loaded in the same reader as the parent entites. For example a code similiar to this could produce this error:
var users = context.Users.Where(u => u.FirstName.StartsWith("Ha"));
foreach (var user in users)
{
Console.WriteLine(user.Address.StreetName);
}
In the 1st line no data is fetched as we only have prepared a Linq query. When we start the foreach a DataReader is opened and collection of users that meets the our conditions is queried but the reader is not closed. Then inside foreach we reach to the Address property of User which is lazy loaded. This lazy load causes a query execution on the same open DataReader and that's when the exception occurs. If i wanted to get rid of the error i could simply add a ToList() (or anything causing the query to perform) to the end of the line like this:
var users = context.Users.Where(u => u.FirstName.StartsWith("Ha")).ToList();
Hope this will help you.
Just add preload reader=true in your postgresQL connection string.
<connectionStrings>
<add name="PostgresQL Npgsql" connectionString="server=srvubuntu01;user id=postgres;password=postgres;database=WinUnified;preload reader=true" providerName="Npgsql" />
</connectionStrings>
Related
I'm using EntityFramework 6.1.3. I have generated models from an existing database. Now, when trying to query the database, datacontext returns null. The database appears to be empty.
But the database is not empty as I can connect and query it using LINQPad. When googling the problem I came across a similar situation. In that case the EF generated an empty database on the local server and the solution was to tweak the connection string, although without clear details what exactly should be changed.
However, I'm not sure if that is the case in my situation.
My connection string is pointing to the remote server.
It is not possible that the database was created on the remote server, as we don't have authorisation.
The database was not created on my local machine as I dont even have local sql server installed. And my AppData folder is empty.
Any suggestions?
Generated connection string below
<add name="MAST_DEV" connectionString="metadata=res://*/Models.Mast_DevModel.csdl|res://*/Models.Mast_DevModel.ssdl|res://*/Models.Mast_DevModel.msl;provider=System.Data.SqlClient;provider connection string="data source=xxxxxx;initial catalog=MASTER_DEV;user id=xxxxx;password=xxxxxx;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient" />
I don't really know what I was doing wrong. I have created simple project from scratch , generated models again and it does work fine now. Possibly, previously I selected 'Code first from the database' from Entity Data Model Wizard. Whereas this time 'EF Designer from the database'. Anyway, thanks for trying to help. It was my mistake
This is the exact solution for my problem (the answer is copied from here Entity Framework cant use the DbContext, model being created)
"I see you are using the EDMX with Templates (.tt) for generating the classes. But if you are getting the information from a existing database, the wizard will create a ConnectionString compatible with ObjectContext (metadata informations and provider of entityframework).
The problem is that the connectionstring you are using is for ObjectContext (Database First and Model First). For the DbContext you should use the connectionstring without the metadata informations"
I am developing an application in which the database is selected by the end user at runtime. The database can either be on a MS SQL server or an IBM DB2 server. I am currently using IBM DB2 10 Express-c on a windows server for testing. I am developing using Visual Studio 2013 C# and Entity Framework 6. I have installed the EntityFramework.IBM.DB2 Nuget package for the DB2 support. I am using reverse-engineer code-first against an existing SQL server database to generate my base code. The application works fine against a SQL Server database.
I am using System.Data.Common.DbProviderFactories.GetFactory to generate the provider.
System.Data.EntityClient.EntityConnectionStringBuilder connectString = new System.Data.EntityClient.EntityConnectionStringBuilder(a_Connection);
System.Data.Common.DbConnection conn = System.Data.Common.DbProviderFactories.GetFactory(connectString.Provider).CreateConnection();
conn.ConnectionString = connectString.ProviderConnectionString;
LB500Database = new LB402_TestContext(conn, true);
a_Connection is provider=IBM.Data.DB2;provider connection string="Database=LISTBILL;User ID=xxxx;Password=yyyy;Server=db210:50000"
and is being parsed correctly by the EntityConnectionStringBuilder.
I then try to access a table in the database with
LBData500.LB_System oneSystem;
System.Linq.IQueryable<LB_System> allSystem = LB500Database.LB_System.Where(g => g.DatabaseVersion == databaseVersion && g.CompanyID == companyID);
I get an invalid operation exception "Sequence contains no matching element" which means that no elements are returned. If I remove the Where so that all rows are returned (there is one in the table) and try to enumerate the result set using the VS debugger I see the message:
"The context cannot be used while the model is being created. This exception may be thrown if the context is used inside the OnModelCreating method or if the same context instance is accessed by multiple threads concurrently. Note that instance members of DbContext and related classes are not guaranteed to be thread safe."
I am not using multi-threading. I am not inside the OnModelCreating.
Just changing the connect string to point to SQL server works fine, so I think my basic approach is sound. If I were getting some kind of error back from the server I would have something to go on. I can run the query from inside Visual Studio, so I have connectivity.
Any pointers would be appreciated.
UPDATE:
I turns out the EF objects were generated using EF5 and the EF6 runtime was being used. I regenerated the EF objects using EF6 reverse engineer code first. I can now connect to the database and get an error message:
"ERROR [42704] [IBM][DB2/NT64] SQL0204N \"DBO.LB_SYSTEM\" is an undefined name."
The schema in the DB2 database is the same as my userid (in this case, not always). I added the CurrentSchema=xxxx to the provide connection string, but EF is still passing dbo as the schema name.
Now I need a way to change the schema name at run time. I saw a link to codeplex EFModelAdapter (http://efmodeladapter.codeplex.com). So I may give that a try.
Update2 After looking through EFModelAdapter, I decided to take a different route. Since I only need database access and not schema management, I decided to go with Dapper (https://github.com/StackExchange/dapper-dot-net). This works great for what I need and allows me to change the schema name when accessing DB2 databases.
As per my Update 2, Entity Framework was a little overkill for what I needed. I switched to dapper https://github.com/StackExchange/dapper-dot-net and I am working fine against multiple DBMSs.
I have one app that uses EnterpriseLibrary and Unity, and uses TransactionScope in just one place. This works nicely, despite the fact that it runs against SQL Server 2005:
// Execute a stored proc using a DbDatabase object inserted by Unity
using(TransactionScope scope = new TransactionScope(TransactionScopeOption.Required))
{
// Update something using the same DbDatabase object
// Run the stored proc above, again
// Assert that the results are different than from the previous call.
}
Yes, this deliberately ends without a scope.Complete(): the example is from a test.
I also have another application just beginning. It uses Entity Framework 4.1. It accesses the same database on the same server. I attempted to use TransactionScope, with the same "make change, verify change, roll back change" idea in mind.
using(TransactionScope scope = new TransactionScope(TransactionScopeOption.Required))
{
using(ProjectEntities db = new ProjectEntities())
{
Assert.IsFalse(db.tblEntities.Any(e=>e.X == desired_value));
db.tblEntities.Add(new tblEntity() { X = desired_value });
db.SaveChanges();
Assert.IsTrue(db.tblEntities.Any(e=> e.X == desired_value));
}
}
This fails with the very familiar error about MSDTC not being enabled for network access.
Right now, this minute, the first test in the first project succeeds, the second test fails.
So I have two questions:
Is there a way to rejigger my second test that would keep the transaction from escalating to MSDTC?
Anybody know why I'm getting different results from the two frameworks? Does EntLib keep a single connection allocated and open during the whole time it's used? Does EF do the opposite?
I have made many test regarding EF, EntLib DAAB, and TransactionScope.
There are several points you must take into account.
SQL Server Version
Connection String
EF and EntLib version
I don't remember the other combinations, but with SQL Server 2008 or later, EF5, and Entlib 5, you can enroll several DbContexts and DAAB operations in the same TransactionScope without scalating to MSDTC. But there's a very tricky part:
the connection string must include: MultipleActiveResultSets=true;
the connection string must have the exact format used by EF
The second part is the most confusing: when you use a connection string to EF, it will chaneg its format, but EntLib uses it as is in the connection string of the config file. So, what you have to do is debug the code, and note down the modified version of the connection string used by EF. You can find it in ctx.Database.Connection.ConnectionString, where ctx is the DbContext you're using. Once you've done so, just copy and paste the modified version of your connection string to your config file, and both EF and EntLib will use the same connection string, thus not escalating to MSDTC.
For previous versions of SQL Server (and sometimes depending on EF version) you can find different problems, but this guidelines can help you test your exact setup.
I don't know about the EnterpriseLibrary but EF does create and open new connection for every query and I think this is why you see those different results.
You can verify this by opening two DbConnections by hand.
I'm trying to use Entity Framework to query database and I have following code that I'm using to get some data.
var students= MyEntities.Students.Where(s => s.Age > 20).ToList();
This code works fine and returns correct data. However, if I run this code, then go to database and update records to change the data this code should return, and then re-run this code without shutting down the app, I'm getting original data.
I'm pretty sure it used to work fine, but now this doesn't refresh data.
No it never worked. This is essential behavior of entity framework (and many ORM tools) called identity map (explanation here).
If you run the query on the same context you will not get your entities updated. It will only append new entities created in the database between running those two queries. If you want to force EF to reload entities you must do something like this:
ObjectQuery query = MyEntities.Students;
query.MergeOption = MergeOption.OverwriteChanges;
var students = query.Where(s => s.Age > 20).ToList();
You are running into issues because EF caches data, so if the data changes behind the scenes, and you dont dispose/reopen your context you are going to run into issues.
The general rule of thumb is to keep the lifetime of the context as short as possible, to circumvent issues like you just mentioned.
Please do not disregard what I said above, but if you would like to force a refresh from the DB you could use the Refresh() method.
This problem is not readily reproducible in a simple example here but was wondering if anyone has any experience and tips, here is the issue:
using Entity Framework
have many points in application where (1) data is written to some entity table e.g. Customer, (2) data is written to history table
both of these actions use Entity Framework, HOWEVER, they use different contexts
these actions need to be both in one transaction: i.e. if one fails to write, the other should not write, etc.
I can wrap them with a TransactionScope,
like this:
using (TransactionScope txScope = new TransactionScope()) {
...
}
but this gives me:
Microsoft Distributed Transaction Coordinator (MSDTC) is disabled for
network transactions.
Our database admin has told me that MSDTC is disabled by choice and can not be installed.
Hence I am making changes trying to create my own EntityConnection with a MetadataWorkspace with the idea that each context will use the same EntityConnection. However, this is proving near impossible trying to get it to work, e.g. currently I continue to get the above error even though theoretically both contexts are using EntityConnection. It's difficult to understand where/why Entity Framework is requiring the MSDTC for example.
Has anyone gone down this road before, have experience or code examples to share?
Well, the problem is quite easy.
If you are using sql server 2008 you should not have that problem because you have promotable transaction, and as .NET knows that you are using the same persistence store (the database) it wont promote it to DTC and commit it as local. look into promotable transaction with sql server 2008.
As far as I know Oracle is working in its driver to support promotable transactions, but I do not know the state, MS oracle driver does not support it.
http://www.oracle.com/technology/tech/windows/odpnet/col/odp.net_11.1.0.7.20_twp.pdf
If you are using a driver that do not support promotable transactions it is impossible for .NET to use local transaction doing two connections. You should change your architecture or convince the database admin for installing MSDTC.
I had a similar problem with SQL 2008, Entity Framework.
I had two frameworks defined (EF1, and EF2) but using identical connection strings to a sql 2008 database.
I got the MSDTC error above, when using nested "usings" across both.
eg the code was like this:
using (TransactionScope dbContext = new TransactionScope())
{
using (EF1 context = new EF1())
{
// do some EF1 db call
using (EF2 context2 = new EF2())
{
// do some EF2 db call
}
}
dbContext.Complete();
}
It wasnt as simple as this, because it was split across several methods, but this was the basic structure of "usings".
The fix was to only open one using at a time. No MTDSC error, No need to open distributed transactions on db.
using (TransactionScope dbContext = new TransactionScope())
{
using (EF1 context = new EF1())
{
// do some EF1 db call
}
using (EF2 context2 = new EF2())
{
// do some EF2 db call
}
dbContext.Complete();
}
I think that what you need to do is to force your contexts to share single database connection. You will be able then to perform these two operations against two different contexts in single transaction. You can achieve this by passing one EntityConnection object to both of your context's constructors. Of course this approach will require you to pass this object to methods which update DB.
I have recently blogged about creating database context scope which will make using multiple EF contexts and transactions easier.