当SQL Azure中不存在数据库时，我是否需要首先迁移EF代码？

Question

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? ie 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?

Answer 1

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, ie 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.

Answer 2

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:

1. 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.

2. 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...