How to recover from Entity Framework nightmare - database already has tables with the same name - entity-framework

Question

How do you get EF back in Sync with code without losing data when Update-database returns the following message
Error Message: System.Data.SqlClient.SqlException (0x80131904): There
is already an object named '' in the database.

Answer 1

I originally wrote this as a self-answering question as I had struggled with the proble for some time, as had a few colleagues, but unfortunately, my answer was deleted and I can't recover it.
Since it's a situation that I suspect can happen several times as people try to "clean up" old migrations, I thought I'd document it with step by step instructions.
Description of the situation we found ourselves in:
We couldn't can't create a new local database because the init script was incomplete, and couldn't apply updates to the production database because the migration scripts create tables that already exist. And, we didn't want to delete production data.
Symptom: Can't run Update-Database because it's trying to run the creation script and the database already has tables with the same name.
Error Message: System.Data.SqlClient.SqlException (0x80131904): There
is already an object named '' in the database.
Problem Background:
To understand this in more detail, I'd recommend watching both videos referenced here:
https://msdn.microsoft.com/en-us/library/dn481501(v=vs.113).aspx
To summarise, EF understands where the current database is at compared to where the code is at based on a table in the database called dbo.__MigrationHistory. When it looks at the Migration Scripts, it tries to reconsile where it was last at with the scripts. If it can't, it just tries to apply them in order. This means, it goes back to the initial creation script and if you look at the very first part in the UP command, it'll be the CreeateTable for the table that the error was occurring on.
Solution: What we need to do is to trick EF into thinking that the current database is up to date, while "not" applying these CreateTable commands since the production database already exists. Once production DB is set, we still need to be able to create local databases as well.
Step 1: Production DB clean
First, make a backup of your production db. In SSMS, Right-Click on the database, Select "Tasks > Export Data-tier application..." and follow the prompts.
Open your production database and delete/drop the dbo.__MigrationHistory table.
Step 2: Local environment clean
Open your migrations folder and delete it. I'm assuming you can get this all back from git if necessary.
Step 3: Recreate Initial
In the Package Manager, run "Enable-Migrations" (EF will prompt you to use -ContextTypeName if you have multiple contexts).
Run "Add-Migration Initial -verbose". This will Create the initial script to create the database from scratch based on the current code.
If you had any seed operations in the previous Configuration.cs, then copy that across.
Step 4: Trick EF
At this point, if we ran Update-Database, we'd be getting the original error. So, we need to trick EF into thinking that it's up to date, without running these commands. So, go into the Up method in the Initial migration you just created and comment it all out.
Step 5: Update-Database
With no code to execute on the Up process, EF will create the dbo.__MigrationHistory table with the correct entry to say that it ran this script correctly. Go and check it out if you like.
Now, uncomment that code and save.
You can run Update-Database again if you want to check that EF thinks its up to date. It won't run the Up step with all of the CreateTable commands because it thinks it's already done this.
Step 6: Confirm EF is ACTUALLY up to date
If you had code that hadn't yet had migrations applied to it, this is what I did...
Run "Add-Migration MissingMigrations"
This will create practically an empty script. Because the code was there already, there was actually the correct commands to create these tables in the initial migration script, so I just cut the CreateTable and equivalent drop commands into the Up and Down methods.
Now, run Update-Database again and watch it execute your new migration script, creating the appropriate tables in the database.
Step 7: Re-confirm and commit.
Build, test, run. Ensure that everything is running then commit the changes.
Step 8: Let the rest of your team know how to proceed.
When the next person updates, EF won't know what hit it given that the scripts it had run before don't exist. But, assuming that local databases can be blown away and re-created, this is all good. They will need to drop their local database and add create it from EF again. If they had local changes and pending migrations, I'd recommend they create their DB again on master, switch to their feature branch and re-create those migration scripts from scratch.