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Apparently IMigrationMetadata.Target encodes the state of the EF model. Can I use this to reconstruct the model for a particular migration?

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  • +1, we want to avoid running migrations automatically and instead run them when an admin invokes them, so we need to be able to reconstruct a model from whatever the current migration is. Commented Mar 29, 2013 at 19:14
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    Could you elaborate a bit? Like where and when would you like to reconstruct the model? What problem would you like to solve? Commented Mar 29, 2013 at 21:26

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Yes, it is possible. I was myself curious what exactly those magic resource strings were storing. By digging into the Entity Framework source (see the DbMigrator.GetLastModel() method), I found out that the IMigrationMetadata.Target just stores a base-64 string containing gzipped XML data. To test this, I created a new console application containing a simple code-first model defined as follows:

public class ContactContext : DbContext { public virtual IDbSet<Contact> Contacts { get; set; } } public class Contact { public int Id {get; set;} public string FirstName { get; set; } public string LastName { get; set; } }

Then I created a migration using the NuGet Package Manager Console:

PM> Enable-Migrations PM> Add-Migration MyMigration

Next I added the following code to my application's Main() method to decode the value in that string and dump it to the console:

var migration = new MyMigration(); var metadata = (IMigrationMetadata)migration; var compressedBytes = Convert.FromBase64String(metadata.Target); var memoryStream = new MemoryStream(compressedBytes); var gzip = new GZipStream(memoryStream, CompressionMode.Decompress); var reader = new StreamReader(gzip); Console.WriteLine(reader.ReadToEnd());

This outputs an EDMX file representing the Entity Data Model associated with my DbContext that created the migration. If I write this output to a file with the .edmx extension, I'm able to open it with Visual Studio and view it in the Entity Designer.

Then if for some reason I wanted to regenerate the DbContext and entity classes that produced the model, I would need only do the following:

  1. Add the .edmx file to a Visual Studio project.
  2. Install the EF 5.x DbContext Generator for C# if I don't already have it.
  3. Add the related T4 templates by selecting Add -> New Item from project node context menu.
  4. Modify the newly added .tt files, replacing $edmxInputFile$ with the name of my .edmx file.
  5. Watch as the two templates magically regenerate my code-first types to their respective .cs files.

Hope that answers your question! :-D

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2 Comments

Cool. Now i just need to think of an actual application for this :D
Great stuff @luksan, thanks! For anyone interested I've created a little Gist that can extract the EDMX from the target hash and compress it back again: gist.github.com/gligoran/87fe3e8eadf5db97ad03. I use this when I need to change a migration without disturbing the rest of the chain. I extract the EDMX from my change migration, edit the XML and compress it back to get the new target. I then have to do this for every migration that follows the changed one.
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I created a small console app to export EDMX from the Model column of the __MigrationHistory table https://github.com/andreydil/EfMigrationModelDecoder
You can choose specific migration using /migration parameter, i.e:

EfMigrationModelDecoder.Cli.exe "<connectionString here>" /migration:Init
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I created a PowerShell script to extract the latest migration from a DB to a edmx-file.

https://gist.github.com/otto-gebb/93d021c8fd300646dba0073a77585a94

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You can also use SQL...

SELECT CONVERT(xml, DECOMPRESS(Model)) FROM [dbo].[__MigrationHistory] WHERE MigrationId = 'NameOfMigration'
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I tried it in my MSSQL 2008,It doesn't work.Warning 'DECOMPRESS' doesn't exist. which version sqlserver you use?
SQL Server 2016 and later. Sorry...I just haven't seen a SQL 2008 server in quite a whiel.

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