I have my database already set, I want to add a new field to the model, a new column to the table, is there a way I can do this without losing all my data?
Normally if you delete the DB it will recreate everything automatically but I don't want to lose the data.
I'm using SQL Server 2008 as database.
You will need to use EF Migrations to add the new column to your database. You can read more about EF Migrations here and here.
If you're using code first, I've always just added the column to the database manually in situations like that. Unfortunately, there is no simple way to automate incremental model updates with Code First.
For example, one of EF Code First's own errors even specify manual update as the best option:
The model backing the ‘your context’ context has changed since the database was created. Either manually delete/update the database, or call Database SetInitializer with an IDatabaseInitializer instance. For example, the DropCreateDatabaseIfModelChanges strategy will automatically delete and recreate the database, and optionally seed it with new data.
Related
I'm new to EF, just a question on DbSet<T>.Add() tracking feature.
I can understand that DbSet<T>.Update() does need to support tracking because EF needs to check which properties has been changed then generate correct update statement to avoid unnecessary updates in non-modified columns.
But why DbSet<T>.Add() also needs to support tracking? Because when we use .Add() method, we want to add a brand new record into the table in database. Since the data model is new, every property should be included in the Insert SQL statement that will be sent to the database, so there is no points to track the newly created object, EF just need to include all properties to generate Insert SQL.
I am using EntityFramework CodeFirst, I have added a relation to one of the tables but when I am trying to update the database from package manager console, I need to add -verbose flag but I have data on my table and need to keep them. Is there a way to change the database without losing data?
Thanks.
You need to manually edit the Up and Down methods of the migration to add relation on the database by adding t-sql commands.
We are using the Code-first approach without an Edmx file, its running fine to create database the first time.
But if I am adding new data entities say new class to my database context then it is not able to add that to new table in that database.
Say for example there are two table initially in database.
ex Database : DbTest
Table : Tbl1, Tbl2
Now if I add new table, say class name 'Tbl3', then it should be adding it into the existing database.
Can any one please explain to me with an example how it can be achieved via code first approach?
I have seen mentioned something like Database.SetInitializer(new ........)
What do I need to put in the blank area of the constructor above?
If you look in your database you will see a table called "EdmMetadata" which Entity Framework uses to determine if any changes have been made to your model since the database was created (which it has in your case).
The default behaviour is for an exception to be thrown if the model and database differ. To get different behaviour you will need to use an IDatabaseInitializer<TContext>.
Luckily, Entity Framework ships with some default implementations of this interface:
CreateDatabaseIfNotExists<TContext> - This will create the database if one doesn't already exist.
DropCreateDatabaseAlways<TContext> - This will re-create the database each time your application is run.
DropCreateDatabaseIfModelChanges<TContext> - This will re-create the database if a change is detected in the EdmMetadata table (usually as a result of creating new tables).
You can of course also create your own implementation of this interface by overriding the InitializeDatabase method.
an example of using one of these initialization strategies is shown below:
Database.SetInitializer(
new DropCreateDatabaseIfModelChanges<NameOfYourDbContextClass>())
Think carefully before choosing an initialization strategy as you could end up losing data already entered into the database and this may not be what you want.
The implementations provided by Entity Framework provide a Seed method for loading your database with data so that you can preload your database with default data each time it is created.
This article provides further information.
Suppose I have an existing database set up using Entity Framework. Is there a mechanism through which I can safely add or remove entities (or their properties) such that the database is altered automatically?
I know there's an option to "Update Model From Database". Is there an equivalent "Update Database From Model" ? Is there a way to configure Visual Studio to do this automatically?
Entity Framework 4.3 has Code First Migration support.
EF helps you with checking the differences between your code and database and then generates code for you that handles this changes. You can use the NuGet package manager console to enable migrations, add a new migration and run them against your database (or create a sql script).
This blog explains how the Migrations work and this blog shows how you can use it with an existing database
Altering the database schema isn't a straightforward operation (has a column been renamed, or is it new column? Can the old type be converted to the new type?) that you can easily infer from the model.
EF doesn't alter the tables for you - it can Drop-Create the DB for you when you change it. However, if you change the existing database by hand to suit the model, EF doesn't seem to mind. It looks like what they check for is Hash(Model) = Hash(Tables).
I want to utilize Elmah in my MVC application to store error messages, and I want to store the exceptions in my application's database. To do that I need to run the included DDL to create the Elmah tables and stored procs.
However, since my development database is recreated whenever my model changes (Via EF CodeFirst) I need the DDL to be run any time the database is recreated.
How would I go about doing this? The only place I could think to put this would be to add calls to run the Sql in the Seed() overridden method in my DbInitializer, but it doesn't seem completely appropriate since I am not seeding elmah, I am creating the structure for the schema to be created.
What is the most appropriate way to apply the DDL upon database recreation?
Using Seed method is usual approach to place custom SQL to execute after database is created. Its main purpose is to fill some initial data but developers use it for creating indexes, constraints, etc. so you can place there anything you need.