Im getting this error:
The model backing the 'database' context has changed since the database was created. Consider using Code First Migrations to update the database (http://go.microsoft.com/fwlink/?LinkId=238269).
But the problem is that my entities are the same as in the database: same maxlength, if null, same data type.
The error occurs when i update my model, since the database and the model have not the same attribute. But my question is. Is there any other case that this error could occur? It says that the error is in this code:
int UserId = (from d in db.Tbl_Users
where d.dbUserID == userName && d.dbPassWord == pass
select d.dbUser).SingleOrDefault();
Everything is its ok, the tables attribute are totally the same(database and model) but when i run the app, the error fire. So, ill repeat the question, When does the error migrations occur? Is there another escenario that the error fire??
It's giving you the error there because it's the first time that you're using the DbContext in your application to call the database. Whenever you ask for database records for the first time in the application Entity Framework will automatically check to see if the model that the DbContext generates is the same as the one already found in the database. Not the database tables themselves, but the model that is stored in the database. When it notices that they are different it automatically throws that exception.
To avoid it you'll need to set some sort of initializer to tell Entity Framework what to do if the models don't match. If you're using migrations it would probably look like this (do this before the first time you call for the context):
Database.SetInitializer(new MigrateDatabaseToLatestVersion<MyDbContext, Configuration>());
There are also other initializers to use, like DropCreateDatabaseAlways and DropCreateDatabaseIfModelChanges, but I much prefer the migration one.
This must be the first time you're using the DbContext class (i.e. connecing to the DB), so that is when the internal data model is built and compared to the database schema.
Edit: you can delete the contents of dbo.__MigrationHistory if you're not using migrations and want to quickly see if it solves the problem.
Related
I have the problem that my database can contain invalid values that come from a previous version of the software. For these values we have to decide individually how to handle them.
I have created a corresponding Entity Framework that can be used to access the database. Internally on the test systems everything worked as well, but during the first test installation on a customer system the program always crashed due to the invalid values in the database.
Now the question is can I detect the errors in the database using the Entity Framework?
I would like to know in which records, which columns lead to errors.
I can't assume any Keys to get each Entity on its own. So my first idea was to check each repository (DbSet) from the DbContext for each entity (QueryingEnumerable). However, with DbSet you can't really access a single entity, you can only go through the query until the first error occurs.
Also, I still don't know which columns led to the error since the exceptions don't contain any information.
Any suggestions what else I can try?
If you have ids or keys for given entities held by dbSet, then you can get single entity.
// In repository or somewhere else, where you have access to this particular DbSet
DbSetOfSomeEntity.FirstOrDefault(x => x.Id == /* Some id number, guid etc */)
Access them in try-catch block, read exception and decide what to do next.
Almost every time I try to update my database with Entity Framework Code-First Migrations, I get the error saying that I need to apply the previous Migration to the database.
So I run the update-database command and this complains that:
There is already an object named ... in the database.
In every case, the object that its complaining about is a table which is literally supposed to be there.
This answer doesn't help since the root problem is the fact that I can't add a migration.
Only way I know to fix this is to physically delete the database from SQL Server and let EF recreate it but that's not always a practical course for obvious reasons.
How do I get around this problem so that I can add the migration?
I have tried lots of variations of EF migration v6.0.1 (from no database, to empty databases to existing databases) and I have a particular problem with Azure DB instances not being able to correctly be created on first deploy using Octopus deploy.
There are a number of places where this could be going wrong, so I thought I would check some basics of EF Code First migration with you fine people if I may:
If I create a code-first model, and know that the database does not exist in the intended target database server on Azure. With the default of 'CreateDatabaseIfNotExists' approach and with AutomaticMigrations disabled;
If I then call 'migrate.exe' with the assembly containing my DbContext and migration Configuration will I get a new database created with the current state of the model? or will I get a new database with nothing in it? i.e. do I need to explicitly 'add-migration' for the initial state of the model?
I have read in the documentation that the database instance should be created automatically by the migration process, but no one states clearly (at least to me) that this newly created database will be generated with the current model without a formal 'initial state' migration created.
So the question is this: do I need an explicit migration model generated for migrate.exe to work from?
Through whatever means I try, I get a database but the application launches with the unfriendly message "Model compatibility cannot be checked because the database does not contain model metadata. Model compatibility can only be checked for databases created using Code First or Code First Migrations." Remembering that this is the same application library that just created the database in the first place (from scratch) I fail to understand how this has happened!
I did manually delete the target database a few times via SQL Server management studio, is this bad? Have I removed some vital user account that I need to recover?
Migrations and the Database Initializer CreateDatabaseIfNotExists are not the same.
Migrations uses the Database Initializer MigrateDatabaseToLatestVersion, which relies upon a special table in the database _MigrationsHistory.
By contrast, CreateDatabaseIfNotExists is one of the Database Initializers which relies upon the special database table EdmMetadata. It does exactly as it implies: Creates a database with tables matching the current state of the model, i.e. a table for each DbSet<T>, only when the database does not exist.
The specific error you have quoted here, Model compatibility cannot be checked because the database does not contain model metadata., occurs due to the existence of DbSet<T> objects which were added to the code base after the initial database creation, and do not exist in EdmMetadata.
There are 4 basic Database Initializers available, 3 of which are for use when migrations is not being used:
CreateDatabaseIfNotExists
DropCreateDatabaseWhenModelChanges
DropCreateDatabaseAlways
Also note, the 4th Initializer, MigrateDatabaseToLatestVersion, will allow you to use Migrations even if AutomaticMigrations is disabled; AutomaticMigrations serves a diffierent purpose, and does not interact with the Database Initializers directly.
If you intend to use Migrations, you should change the Database Initializer to MigrateDatabaseToLatestVersion and forget about the other 3. If, instead, you intend to not use Migrations, then the choice of Initializer is situational.
CreateDatabaseIfNotExists will be more appropriate when you are certain that your data model is not undergoing active change, and you only intend to be concerned with database creation on a new deployment. This Initializer will elp ensure that you do not have any issues with accidental deletion of a database or live data.
DropCreateDatabaseWhenModelChanges is most appropriate in development, when you are changing the model fairly often, and want to be able to verify these changes to the model. It would not be appropriate for a production server, as changes to the model could inadvertently cause the database to be recreated.
DropCreateDatabaseAlways is only appropriate in testing, where your database is created from scratch every time you run your tests.
Migrations differs from these 3 Database Initializers, in that it never drops the database, it instead uses Data Motion to execute a series of Create Table and Drop Table SQL calls.
You also can use Update-Database -Script -SourceMigration:0 in the Package Manager Console at any time, no matter which Database Initializer you are using, to generate a full SQL script that can be run against a server to recreate the database.
Firstly, many thanks to Claies who helped me get to the bottom of this problem. I have accepted his answer as correct as ultimately it was a combination of his response and a few additional bits of reading that got me to my solution.
In answer to the actual posts question 'Do I need a migration for EF code first when the database does not exist in SQL Azure?' the answer is yes you do if you have disabled automatic migrations. But there is a little more to be aware of:
The Azure aspects of this particular problem are actually irrelevant in my situation. My problem was two-fold:
The migration being generated was out of sync with respect to the target model. What do I mean? I mean, that I was generating the migration script from my local database which itself was not in sync with the local codebase which created a migration that was incorrect. This can be seen by comparing the first few lines of the Model text in the __MigrationHistory. This awareness was helped by referring to this helpful post which explains how it works.
And more embarrassingly (I'm sure we've all done it) is that my octopus deployment of the web site itself (using Octopack) somehow neglected to include the Web.Config file. From what I can tell, this may have occurred after I installed a transform extension to Visual Studio. Within my nuget package I can see that there is a web.config.transform file but not a web.config. Basically this meant that when the application started up, it had no configuration file to turn to, no connections string at all. But this resulted in the slightly misleading error
Model compatibility cannot be checked because the database does not
contain model metadata.
Whereas what it should have said was, there isn't a connection string you idiot.
Hopefully this helps people understand the process a little better after reading Claies answer and also that blog-post. First though, check you have a web.config file and that it has a connection string in it...
I use a third party database with Entity Framework 6. This works fine; however, when a table within my model is changed (three columns were deleted), my program throws an exception:
System.Data.SqlClient.SqlException: Invalid column name '<deleted column>'
I don't use any of these columns. I only read them from the database.
I can update my model, but then when there is another change in a table, my program will crash again. How can I modify my program so that it won't crash on the next database change?
You can use a Code First approach starting from database (generate classes from database). At the end of class generation you can delete entities that you don't need (i.e. entities related to all unused tables) or properties related to unused fields.
Disable migrations.
You can also delete intermediate files generated by EF code generation (files different from .cs files).
At this point, any changes to database that not affects mapped classes/properties does not cause errors in EF.
Create a context with only the entities you need. Create entities with only the properties you need.
see EF code first.
Use Fluent API to specify primary keys and more.
It will still crash though if any of your entities/properties gets changed/deleted
I am using Entity Framework to save some data in a SQLite database file.
This is my table in DB file
ID Name Username Pwd
ID is autoincrement, and rest of columns are of Text type
This is how I am saving my data into the database:
UserInfo userInfo=new UserInfo();
userInfo.Name="abc";
userInfo.Username="xyz";
userInfo.Pwd="123456";
using (var context = new ApplicationContext())
{
context.UserInfo.Add(userInfo);
context.SaveChanges();
}
The problem is, this code is not inserting any new row into the table. I also tried this before saving, but no luck
context.Entry<UserInfo>(userInfo).State = EntityState.Added;
context.ChangeTracker.DetectChanges();
I tried debugging and no exception is occurring.
How can I insert new row into a table using Entity Framework?
Possibilities that may cause an issue.
Right Click on your Entity Data Model and Update Model From Database. To make sure you have updated till the last change.
Make sure you are looking at the correct Database and Table. Every one makes this mistake once in a while.
Make sure you are using the valid connection string. Pointing the proper databases. Make sure its not a old db / some other backup db.
Make sure you are using the proper entity which is up to date.
The above code you have, its perfectly working fine to me. I just did a workout to sort out this issue.
The above points are guesses.
Refer this Link Entity Data Model Example & One More Example Code available here
I had a similar problem once and it can be a pain to determine what exactly causes this. One issue that might be causing such a behavior in EF is having multiple instances of the Context object such that when you call SaveChanges on your Context, you are actually calling it on a different instance than the one you added the entity on (and EF does not detect any changes in the entities or the new entity is not attached to the right context, causing SaveChanges to not send any SQL requests to the database).
I suggest debugging this in VS (using the object id feature) in order to see if you have multiple context instances. Using the Unit Of Work pattern together with repositories is a way to have a better control over the lifetime of the context objects in your application