How to solve migration issues with rails
Greetings friends!!!
Migrations make managing your database schema easy, however, problems often occur when we're running our migrations and run into a bug in our migration code. It can leave your migrations in a broken state where you can't migrate up and you can't migrate down. It's important to learn how this happens and how to resolve these problems when they occur. Use rails db:migrate version 0 to migrate to 0 level.
STEPS to recover migrations issue:
So it's going to run the first migrations successfully if any and then it's going to get to migration which has a problem. You can see it gives you an error, all later migrations cancelled. It should tell you the problem. Okay, so rails did run the first migrations successfully and it will see those as being complete. Go into mySQL so you can see this.
Okay in there take a look. Do see tables, table schema. You'll see that it did successfully for first migrations. You can actually look at the content of schema migrations. Select * from schema migrations. You can see that it did store the fact that it ran those first successfully. And you notice that my table is not called users anymore. It's actually called admin users. So it did this step right here. Rename table users to admin users. If we take a look we'll see that it actually added this column username and it changed the column email to have this definition.
It went ahead and did all of the steps up until the broken step. So let's go back now. Let's exit out again. Exit and let's go into our code and let's fix our problem. So now we're just going to make this password again, which is correct. And let's try to fix it. Let's run rails db:migrate. So we try and run the migration code up. And we still get an error. Now the error this time is different. The error this time is that the table admin users already exists.
So when it tried to rename the table which was the first step in this up code it couldn't do it. Okay, let's try migrating down again. Migrate version and let's do down and we get another error. This time it had unknown table users for drop table users. Right, it didn't even try to run this method. It tried to come over here and run this one and drop this table but it couldn't because that table doesn't exist anymore. We renamed it. So we're stuck in this state where we're in between the two migrations.
We can neither go up or down. And fixing the bug in our code doesn't solve the fact that we're now in this in between migration state. So we're kind of boxed in. To resolve it we have to get our database to either the upstate or the down state. We don't have any code to move it through half states. One solution would be to open up my SQL and to use my SQL commands such as drop table and then edit the contents of schema migrations to match what's there and what's not there. Not that would work but we're not going to do it that way.
Instead, let's solve the problem just by editing our migration file. The problem is that we got halfway through this migration. The first three lines ran successfully. We can't run them again. So what do we need to do? We should comment them out. And the way that we can comment them out is by putting a pound sign in front of them. So all of those lines are now commented out so when I try to run this up migration now the first thing it's going to do is run this command which is the one we want it to start with. Let's save the file. Let's come back over and let's run rails db:migrate.
Now it completes the rest of the migration as we would want. And it stores the fact that it successfully completed it. So the migration is considered in place. Of course, we don't want to leave these commented out. That was just to fix our problem. So let's un-comment those now. Save the file again and now we should be able to migrate back down and migrate back up again. So the best solution is to go into your migration file temporarily comment out the part that's giving you trouble so you can get the migration to either an up or a down state.
And then you can use your migrations normally again. It's also good to keep your migrations small and concise. Don't overload them with too many changes or you'll create more of these kinds or problems for yourself. Instead, break up each one of the migrations into small pieces. You can have several method calls in one migration. Especially if they're related changes. But for example, you would never want to create four tables inside one migration. Have four different migrations for it. Finally, make sure that all migrations are working 100% correctly in your development environment before you migrate to your production database.
Your production data is sacred and you never want to make migration mistakes that might cause you to lose production data. So get those issues resolved in development first.
Migrations make managing your database schema easy, however, problems often occur when we're running our migrations and run into a bug in our migration code. It can leave your migrations in a broken state where you can't migrate up and you can't migrate down. It's important to learn how this happens and how to resolve these problems when they occur. Use rails db:migrate version 0 to migrate to 0 level.
STEPS to recover migrations issue:
- Make a note of migration issue.
- Try to resolve it manually by changing migration file.
- Perform migration to version 0. This will rollback all migrations.
- Comment all above code inside migration having an issue. Let's say for example if your line # 5 is causing an issue comment your code till line #4.
- Run rails db:migrate again
- Verify that it shows success and double check with the database too, If it showing migration error perform step 1 to 5 till it shows the success.
- Uncomment your code and you are good to go. Make sure that you uncomment your code before you forget else migration won't work for others.
So it's going to run the first migrations successfully if any and then it's going to get to migration which has a problem. You can see it gives you an error, all later migrations cancelled. It should tell you the problem. Okay, so rails did run the first migrations successfully and it will see those as being complete. Go into mySQL so you can see this.
Okay in there take a look. Do see tables, table schema. You'll see that it did successfully for first migrations. You can actually look at the content of schema migrations. Select * from schema migrations. You can see that it did store the fact that it ran those first successfully. And you notice that my table is not called users anymore. It's actually called admin users. So it did this step right here. Rename table users to admin users. If we take a look we'll see that it actually added this column username and it changed the column email to have this definition.
It went ahead and did all of the steps up until the broken step. So let's go back now. Let's exit out again. Exit and let's go into our code and let's fix our problem. So now we're just going to make this password again, which is correct. And let's try to fix it. Let's run rails db:migrate. So we try and run the migration code up. And we still get an error. Now the error this time is different. The error this time is that the table admin users already exists.
So when it tried to rename the table which was the first step in this up code it couldn't do it. Okay, let's try migrating down again. Migrate version and let's do down and we get another error. This time it had unknown table users for drop table users. Right, it didn't even try to run this method. It tried to come over here and run this one and drop this table but it couldn't because that table doesn't exist anymore. We renamed it. So we're stuck in this state where we're in between the two migrations.
We can neither go up or down. And fixing the bug in our code doesn't solve the fact that we're now in this in between migration state. So we're kind of boxed in. To resolve it we have to get our database to either the upstate or the down state. We don't have any code to move it through half states. One solution would be to open up my SQL and to use my SQL commands such as drop table and then edit the contents of schema migrations to match what's there and what's not there. Not that would work but we're not going to do it that way.
Instead, let's solve the problem just by editing our migration file. The problem is that we got halfway through this migration. The first three lines ran successfully. We can't run them again. So what do we need to do? We should comment them out. And the way that we can comment them out is by putting a pound sign in front of them. So all of those lines are now commented out so when I try to run this up migration now the first thing it's going to do is run this command which is the one we want it to start with. Let's save the file. Let's come back over and let's run rails db:migrate.
Now it completes the rest of the migration as we would want. And it stores the fact that it successfully completed it. So the migration is considered in place. Of course, we don't want to leave these commented out. That was just to fix our problem. So let's un-comment those now. Save the file again and now we should be able to migrate back down and migrate back up again. So the best solution is to go into your migration file temporarily comment out the part that's giving you trouble so you can get the migration to either an up or a down state.
And then you can use your migrations normally again. It's also good to keep your migrations small and concise. Don't overload them with too many changes or you'll create more of these kinds or problems for yourself. Instead, break up each one of the migrations into small pieces. You can have several method calls in one migration. Especially if they're related changes. But for example, you would never want to create four tables inside one migration. Have four different migrations for it. Finally, make sure that all migrations are working 100% correctly in your development environment before you migrate to your production database.
Your production data is sacred and you never want to make migration mistakes that might cause you to lose production data. So get those issues resolved in development first.
Comments
Post a Comment