Inside of OnModelCreating, I want to be able to ignore a column if the database is on an older migration EF Core 5 throws an exception if I attempt to read from the database directly, or indirectly by querying the applied migrations. I'm not certian that it's even a good idea, since OnModelCreating is used during the migration đ©, but I'll burn that bridge when I cross it.
There are some examples on how one would do this with EF6, but they don't seem to apply anymore with EF Core.
While Ivan Stoev is right that --generally-- you should model the target database without outside input, the real world isn't always that clear-cut. In my particular case, there are multiple service instances (Azure Functions) that need to read and write to a single database. In order to maintain zero downtime, those Functions need to not read or write columns that don't yet exist.
I solved the problem the way Serge suggested. The database has a known version, populated with seed data that increments with every migration. On startup, the service reads that version with a regular old Microsoft.Data.Sql.SqlConnection. This version is then added to the IServiceCollection as a singleton to be used by the DbContext constructor.
When talking to an older database version, OnModelCreating does things like this:
builder.Entity<Widget>(w =>
{
// another option would be to use the migrations table instead of an integer
if (DatabaseVersion < ContextVersions.WidgetNewPropertyAddedVersion)
{
w.Ignore(w => w.NewProperty);
}
else
{
w.Property(w => w.NewProperty)
.HasDefaultValue(0);
}
});
The startup code also detects if it's been started by the Entity Framework tools and does not read the database version, instead assuming "latest". This way, we do not ignore new properties when building the migration.
Figuring out how to let the service instances know that the database has been upgraded and they should restart to get the new database model is an exercise left up to the reader. :)
Related
I have someone else's ASP.NET application that uses code-first EF and comes with the code for database migration and seed. I usually run migration.exe before the new version of the app gets installed and it worked well as long as I let EF do everything starting from creating a brand new database. Now I have to make it work against an existing and EMPTY database. When I run migration.exe it doesn't return any errors and perform all the required migration steps but it not longer runs seed step (it probably assumes it's not needed since the database already exists). How do I force running seed step in this case?
There are steps you need to ensure are set for your database initialization to achieve what you want:
CreateDatabaseIfNotExists: This is default initializer. As the name suggests, it will create the database if none exists as per the configuration. However, if you change the model class and then run the application with this initializer, then it will throw an exception.
DropCreateDatabaseIfModelChanges: This initializer drops an existing database and creates a new database, if your model classes (entity classes) have been changed. So you don't have to worry about maintaining your database schema, when your model classes change.
DropCreateDatabaseAlways: As the name suggests, this initializer drops an existing database every time you run the application, irrespective of whether your model classes have changed or not. This will be useful, when you want fresh database, every time you run the application, like while you are developing the application.
Custom DB Initializer: You can also create your own custom initializer, if any of the above doesn't satisfy your requirements or you want to do some other process that initializes the database using the above initializer.
The best help guide I followed with getting started on understanding Code first was written here and references the part you are referring to:
http://www.entityframeworktutorial.net/code-first/database-initialization-strategy-in-code-first.aspx
The important thing though is to know that you may want different things for different environments too and may wish to get into understanding how to do different deployments for different environments.
An example of a custom one would be like this:
public class EasyContext : DbContext
{
public EasyContext() : base("name=EasyEntity")
{
Database.SetInitializer<EasyContext>(new EasyInitializer());
}
public DbSet<Person> Person { get; set; }
}
You should be setting a breakpoint to make sure it is being caught or else you can force a new configuration to fire as well.
currently I am working at a project which requires to be backwards compatible with (non-EF) databases, but also want to create a new database from model.
For this task I save the current schema somewhere (in XML form) and update the databases with raw sql update steps, until they match the schema, which is working fine.
Also, the modelBuilder matches the schema (as in, my algorithm finds no difference between the newly created database by context.Database.Create() and my saved schema) currently.
Since the schema will most likely change in later stages of development, I do have to support two ways to create an Up-to-date database and was wondering if I could combine these two - since now I have to update the saved target schema, create the update steps AND update my modelBuilder so that is creates exactly the database I need - which would be quite a tedious task.
So since there is probably no way to "translate" my schema to modelBuilder entries and because there is more not mapped information in my POCO classes (which prohibits the approach of updating a correct database and update my classes database first) the only (visible to me) way would be to somehow gather the CREATE TABLE statements a context would create when I call Database.Create() which I can use to update my schema and the update steps accordingly.
I know quite sure I can do the same by logging the context while calling the Create() method, however - this will take quite some time, will issue some queries I do not need and will create a dump database I have to get rid of afterwards each time I update my model.
So I was wondering if there was a way to inspect the modelBuilder (or the context, of course) and somehow see what the tables would look like it maps to.
My database model (sometimes referred to as "context") is dynamically assembled at startup based on which services and/or plugins are installed. Plugins and services export their model definition fragments through my IoC container and the application core picks them up and runs them when the DbContext.OnModelCreating method is called.
The question is: Can I (and how do I) use Code First Migrations with this setup?
(below is more information on what I've tried and what particular problems are)
In my previous project, the database was inherited from some old code so we couldn't use any of the Code First database generation stuff anyway. We simply kept a long line of delta scripts and executed them manually on deploy (it was a single-host kind of project).
Now I'm starting a new project, and this time, the database is brand new, ready for Code First to play with. Initially, I was all excited about Code First Migrations, seemed like the way to go. Until I actually tried it. The initial attempt, quite obviously, failed due to the absence of an explicitly defined DbContext in my project.
So far, it looks like the only viable option is to manually code migrations, with which I am perfectly fine. However, it turns out that this is not as simple as just creating a few classes inherited from DbMigration.
After some experimentation on a small test project, I was able to find out that the migration autogenerator adds an implementation of IMigrationMetadata, which, among other things, contains a hash of my model as the values of the Source and Target properties. Presumably, this hash is then used to identify a path across migrations from the "current" state of the database (as recorded in the __MigrationHistory table) to the newest state as defined by the model in code. This totally makes sense, but...
Naturally, I have no idea where to get that hash for my model, which makes me unable to implement IMigrationMetadata on my migrations.
On the other hand, I see that the metadata interface is not included in the DbMigration class itself, which makes me think that it might be optional. It then follows that migrations can actually work without the hash values, but the question is - how?
All the information I could find on the internet is just simple, very basic tutorials. No information on how to create migrations manually (and whether it's even supported). No documentation on how it actually works and how to extend it. And it is not quite obvious from outside.
I am ready to resort to ILSpy at this point, but the whole EF is so complex that I fear I may not be able to find what I need in reasonable time.
Here are a few ideas that you could pull together to find a solution that works for you. I realize I mentioned some of these in our other thread, but I'm including them here for others reading this question.
Automatic migrations allow Code First to automatically calculate and apply changes to the database.
You can write your own code to generate and apply migrations. I've written a post about applying migrations and the MigrationsScaffolder class will help you create migrations.
When you run the project , an extra table is created in the database.
EdmMetadata table
The hash is always created with the help of EdmMetadata Entity and the current code first model. It is SHA-256 hash stored in the EdmMetadata table of the database. You can get it from that table.
Methodology to be followed will be:
Get the hash of the current model using
var hash=GetModelHash(OldContext);
Check whether the model in the code (new model) is compatible with the model in database(old model) using
CompatibleWithModel(hash,CurrentContext,ObjectContext)
This method returns bool.
If it is not compatible, then delete the existing tables in the database.
Create new tables
Save the current hash to the databse
Seed the data.
The code may look like:
{
var objectContext = ((IObjectContextAdapter)context).ObjectContext;
var modelHash = GetModelHash(objectContext);
if (CompatibleWithModel(modelHash, context, objectContext))
return;
DeleteExistingTables(objectContext);
CreateTables(objectContext);
SaveModelHashToDatabase(context, modelHash, objectContext);
SeedData(context);
}
Be sure to make the class inherited from
IDatabaseInitializer<T> where T:DbContext
I want to programmatically create the database create script for my EF 4.3 Code-First database. The database creates just fine using the standard DatabaseInitializer mechanisms, but the following code fails:
using(var dc = new MyContext())
{
var objContext = (IObjectContextAdapter)dc;
var script = objContext.ObjectContext.CreateDatabaseScript();
}
The exception I get is:
The store generated pattern 'Computed' is not supported for properties that are not of type 'timestamp' or 'rowversion'.
I do have a Computed column defined of type "string" but like i said, the database creates fine when created via the built-in DatabaseInitializer. Oddly enough, the resulting schema using this method doesn't actually create a computed column.
As to why I'm doing this, I have a script that runs post-create that drops this column and creates a bonafide computed column. Without specifying the column as computed in the EF mapping, it will attempt to assign that column a value upon inserts, which then fails.
Leaving database creation aside for a moment, setting âComputedâ means that EF will not attempt to write the value of that column but will read the value that was (presumably) computer by the server each time the column is queried. From an EF perspective it is valid to have a computed column for any data type.
Now consider database creation from the EF model. EF doesnât know what to do to create the computed column in the database, and indeed it may not even be possible to do it without triggers in certain databases depending on the type of the column. So in EF4 the decision was made to make CreateDatabaseScript throw in these situations.
However, EF 4.3 and later donât use CreateDatabaseScript anymore when targeting SQL Server or SQL Server Compact. They instead use the Migrations pipeline. For the Migrations pipeline we decided to take a different approach because we felt that it was wrong for database creation to always throw for what could be a perfectly valid model. Especially considering you could have written your own SQL in the migration that added a trigger or some other mechanism to make the database create a valid computed column.
So this is why you see the database being created by EF 4.3 (but without doing anything to make the column computed) but then you see the same model throw when trying to use CreateDatabaseScript which uses the older, non-Migrations mechanisms.
The way to fix this is to have Migrations create the scripts instead of using CreateDatabaseScript.
What is the best practice for upgrading the database using ORM (DevExpress XPO, NHibernate or MS Entity Framework)?
I'm starting a new project and have to pick an ORM. The development process requires of releasing intermediate test builds quite often and likely that each build will have changes in the database structure. Each new version has to upgrade the DB gently to keep current data.
For old solutions I would provide a set of SQL scripts for upgrading the database from v1 to v2, from v2 to v3, etc. and execute them sequentially.
But how is it going to work for ORM? Should I still write SQL scripts to upgrade the DB?
I understand that simple adding new fields wouldn't cause a problem (e.g. see UpdateSchema() method for XPO), but what if I have to split a table and reallocate current records into 2 new tables?
I can't comment on the other ORM's, but I have used DevExpress XPO for a corporate treasury application since 2007. The schema changes a little with every release but there have also been some big schema changes over the years as well. A somewhat extended version of the default XPO upgrade mechanism has comfortably catered for all the changes.
There is good basic information here about upgrading XPO applications.
DevExpress provide a DBUpdater tool to assist you with the task of upgrading production environments. You can extend this tool to cater for additional requirements. In my application, we have added some options for logging, preview with rollback, etc.
Each module has virtual UpdateDatabaseBeforeSchemaUpdate() and UpdateDatabaseAfterSchemaUpdate() methods. You can significantly control the upgrade process within these.
As you mention, some of the upgrade will be handled automatically by XPO (e.g., adding a new column), but some things need additional control such as initialising the new column with a default value for existing records.
For instance, let's say MyNewField has been added to the MyEntity XPO class in version 2.0 of your application. Let's say it should default to a value of 3 for existing records. XPO will handle the creation of the new column but existing records will be NULL. (If you specify a default value in the XPO class it would only pertain to new records). In order to correct the value for existing records you would add something like the following to entity module's overridden UpdateDatabaseAfterSchemaUpdate():
public override void UpdateDatabaseAfterUpdateSchema()
{
base.UpdateDatabaseAfterUpdateSchema();
if (CurrentDBVersion < new Version(2, 0, 0, 0))
ObjectSpace.GetSession().ExecuteNonQuery(
"UPDATE [MyEntity] SET [MyNewField] = 3 WHERE [MyNewField] IS NULL");
}
(You could also use ObjectSpace.GetObjects<MyEntity>() and a foreach if you prefer to avoid the direct SQL.)
In your more extreme example of splitting a table in two, you can use the same method, but you would override UpdateDatabaseBeforeUpdateSchema() instead, run the SQL to split the table, let XPO perform any other schema updates and, if necessary, populate any default values in the UpdateDatabaseAfterUpdateSchema().
You will find that you bump into constraint problems e.g., foreign key violations so you might find you need to write some general routines such as DropAllForeignKeyConstraints() as part of the UpdateDatabaseBeforeUpdateSchema(). Sometimes you find that XPO already provide something, sometimes not. Missing constraints and indexes will get regenerated in the schema update. (In my experience switching a master data table's primary key turned out to be the hardest update routine to get right.)
By default the calls all happen in an SQL transaction so if anything fails it should all roll back.
The developers need to be aware of when a change to the domain model is likely to cause a problem with the underlying schema.
For testing, we keep a few old customer databases and run a bunch of before-and-after tests as part of the build process to make sure that existing customers are able to upgrade properly whatever version they are upgrading from. In production whenever we run into a problem upgrading, the problem data is added into this test library to prevent similar problems in the future.
We are dealing with major international companies and banks. The customers are quite happy with the result. In situations where a corporate's DBA needs to sign off on the changes, they don't seem to mind having a command line tool to do the upgrade rather than a script.
Most migration solutions can handle easy tasks, like adding new column, relationship or removing one, but fail to work when you rename a column (is that an add? or a remove following an add which equals a rename? What should you do with the data in that case?)
All three solutions have basic migrations support, XPO even lets you run your own scripts as a part of the process (to insert static/test/contant data, etc.)
There's also the MigratorDotNet project that you can use and not to rely on any ORM specific feature regarding migrations.
Personally, I would use auto migration only in dev/test environment and would have full set of upgrade scripts when running on client specific database to say upgrade from v1 to v2.
How is it going to work for ORM? Should I still write SQL scripts to
upgrade the DB?
Clear answer of this question should be on Programmer's stackexchange thread - What are the criteria for evaluating an ORM for.NET?, there i got simple answer for your question that you asked and matches with my experience with ORM while developing some project with Entity framework and Code smith ORM templates.
How does the ORM manages changes in the data model? what if I have to split a table and reallocate current records into 2 new tables?
Some can update the DB automatically within a certain measure, other
don't do anything and you'll have to do the dirty work yourself; other
provide a framework for handling change that lets you control database
updates. That means every couple of days someone needs to spend an hour updating the model to add a table or change datatypes that are changing
Ref:
https://softwareengineering.stackexchange.com/questions/6543/what-are-the-benefits-of-using-database-abstraction-by-orm
https://softwareengineering.stackexchange.com/questions/41739/best-arguments-for-against-introducing-orm-technology-into-a-companies-dev-proce/41833#41833
If you ask - what is the best practice for upgrading the db using ORM - my answer is: Don't use it if your application is more than a hobbyist app.
There are a lot of scenarios where many ORMs are unable to provide support to your specific database needs, e.g. in creating stored procedures, create indices and views or even indexed views/materialized tables without writing sql scripts. Problems like adding a new non-nullable column to an existing table are much harder to solve in ORM-Migration-Code than by writing SQL scripts.
Current Tools like Visual Studio Data Tools do handle these kind of problems way better.