I'm using Entity Framework 6 Code First, and would like to create a Trigger.
How do I do this?
The reason I need the trigger is because a user may either edit the database directly or through a program I'm writing, and I need to make sure 2 columns in a table are not both null, and are not both not null.
I've been looking and can't find a way.
Is there any way to specify a trigger using code first?
Entity Framework has no support for triggers, although you can certainly manually execute a statement that would create a trigger, but you would need to do this after the table was created (if using migrations).
You can use the technique specified by Ladislav in EF 4.1 code-first adding a trigger to a table
Take note of his warning, however, EF will not be aware of any changes made in the trigger. If your intent is merely to ensure that 2 columns in a table are not null, you'd be better served with a constraint (constraints are also not supported by EF, but you can add them manually).
Check out my library EntityFramework.Triggers. It works at the Entity Framework layer, so the trigger events won't fire if someone modifies the database directly. The NuGet link is https://www.nuget.org/packages/EntityFramework.Triggers/
After you add a migration, open the migration file and create your trigger as shown below
Note: you need to run update-database to see the changes in your database.
Related
I renamed a few tables and some columns. When I run the Add-Migration command, the migration generates code that drops the old tables and columns and adds ones with the new names. This results in losing the data they contained.
Since I don't want to lose the data, I want to edit the migration, removing the drop and add commands, and replacing them with rename commands.
But after I edit a migration, how do I apply that change?
If I run the Update-Database command, that applies it to the database. But not to the snapshot that Entity Framework maintains of my schema (stored in ApplicationDbContextModelSnapshot).
I need a way to incorporate my edits into the model. How can I accomplish this?
So, this is definitely the messy part of code first.
As far as the question asked, as GuruStron suggested, the only way I found to have a valid custom migration is to edit it such that the result is the same as what the original, generated migration produced. This keeps it up to date with the database snapshot. And running Update-Database will run your custom update code.
I think my biggest problem was that I had too many changes going on at once. After struggling with this for a while, I undid some of my changes and added them back bit-by-bit. Entity Framework will rename a table or column if it can figure out that the new name refers to the same column. If it finds many changes, it can't figure this out.
In the end, I had to customize the migration a little for a couple of columns that were being dropped (customized them to be renamed instead). But I was able to get Entity Framework to rename my tables and other columns.
The key: make small changes at a time and carefully review the migration before applying them to the database. If you need to customize the migration, do it such that the end result doesn't change.
You don't.
I suppose you are developing using a code first approach, since the question has this tag on it.
If you are using code first, you must change your models and let Entity Framework change the database schema for you.
Suggested reading:
Migration in Entity Framework Core
Entity Framework Core Migrations
I am working on a project which is using Entity Framework code first for its data structure. It was created with code first, but was never migrated again and only has its initial migration data stored. Since then, the database has been modified directly through server explorer in VS2015.
There is no migration information about any changes and the database has critical information which I cannot lose.
Which brings me to my Questions.
If I create a new migration and update the database from it, will it wipe all changes which were not recorded in migrations and still leave the changes which were made as well?
The details of your question is a bit sketchy, but I will make some assumptions in order to help you along. Please correct where I am wrong.
I assume that you want to keep the data which resulted from the changes which were effected directly to the database, but you do not want to keep the changes that was effected to the database - in other words: keep the data but not the datastructures.
My advice is as follows
Always perform a full backup of your database when you are about to do something you are uncertain about.
If you can identify the tables you want to update, you can always use the SELECT INTO statement to create a quick backup of the specific tables only. These tables will not be removed when you do a EF database migration unless you explicitly script the deletion.
You can build the SELECT INTO statement into your EF migration via the Sql() method, or you can manually run the command against the database.
More information:
Click here to learn about EF code first migrations in general
Click here for a comprehensive code first migration reference
I believe following two posts will help you.
EF 4.3 Migration Walkthrough : http://blogs.msdn.com/b/adonet/archive/2012/02/09/ef-4-3-code-based-migrations-walkthrough.aspx
update:
Code First Migrations with an existing database
https://msdn.microsoft.com/en-us/data/dn579398.aspx
I have redesigned my class scheme many times since started the project I'm working on.
At first I didn't know about data annotations, so I have migration1, where my Ids have no DatabaseGenerated.Identity option and migration4 where they already have it.
Turns out that EF doesn't work this way (https://entityframework.codeplex.com/workitem/509 as for EF5), so when I added some seed data, it threw an exception
Cannot insert the value NULL into column 'primarykeycolumn', table 'tablename'; column does not allow nulls. INSERT fails.
What's really interesting is when I deleted all existing migrations and scaffolded new one from scratch, seed method ran without any problems.
So I have a question: do I need to do it any time I make changes to scheme such as adding data annotations like Identity or is there a way to save my previous migrations? Because dropping and recreating db in real-life can result in a huge data loss, which I want to avoid.
Adding Migrations. In your Package Manager Console
Add-Migration Test1
In your Configuration.cs from Migration Class You will see Up method and Down Method. It is like Before, and After Changes. You can call them up if you want to reverse the Changes.
doing Add-Migration "NameoftheMigration" does not implement it
you need to call
Update-Database
If it tells you that there is an error, or potential Data lost.
Update-Database -Force
If you choose to use CodeFirst. You can't really Jump from Altering table by Design, then by code. You have to Choose one. As CodeFirst Migration rely on Models. Data First, Rely on Data Design.
I am working on a project using Entity Framework 5 with MVC5. My Project is currently running.
I am trying to add a column in a table. But as we know that in EF when we add a field in model it drop and recreate the database, which i can`t do.
One method for this is found code migration. But my manager is not allow me to use that(because its a big database project).
Please help me and suggest something for it.
When I start using code first with Entity Framework, I was in the same situation as you. I was always running Update-Database -F and then watching all my tables get dropped and recreated, even for something as simple as renaming a field.
Versioning databases is hard, but it's much easier with named migrations (which I think it what you mean when you refer to code migrations). I know your boss is against the idea, but it's very flexible.
Essentially you run Add-Migration -Name xxx in your Package Manager Console and Entity Framework will scaffold a configuration class for you with the default commands (both for versioning Up() and Down()) it will execute when you Update-Database. If you don't like the commands, you can change them. You can even move data around if you need to (it's a bit fiddly though).
I think you have four options available to you;
Use code-first automatic migrations: This is what you have at the moment, and doesn't give you enough control over what happens when you update your database. It's good for getting started in the earlier stages of a project, but becomes unwieldy after production.
Use code-first named migrations: Gives you the control you need via Configurations - but your boss has prevented use from using it.
Use a database-first approach: Database First allows you to reverse engineer a model from an existing database. So if you need to make a change, you would change your database first, and then regenerate your models using EF. This is usually favoured by DBA's, but it may mean that you have reimplement some aspects of your existing project.
Dont use entity framework: It's possible that you could revert back to SQL queries, which your boss might accept and gives you the flexibility you need - but who needs that kind of pain?
Let me know if I can help further.
Does anyone knows if it is possible to just rename a database table with Code First in Entity Framework 5?
I just have simple models and a database initializer, but since my database already contains data I've commented that initializer out. So, I don't want to drop and create a whole new database.
When I add new columns to such a model I use the Package Manager Console and run update-database. to update my database. Does this also work for renaming tables? Does the update-database command automatically knows what the old name was?
I don't have a lot of experience with CF yet and learning new stuff every day. Without CF it's 'easy' to just run an alter table command and refresh the dbml file... But with CF? I don't have a clue!
use
<Table("TableName")> _
Public Class ClassName
....
Obviously the update-database command knows what the old name was... So, in case of data loss warnings, you may choose to apply the -force option.