Recreating a table in Rails - ruby-on-rails

I'm using Rails and Postgres. I currently have a Like table created. I need to drop this table and re-create it. Here's what I've done so far.
1) Rename create_likes migration file to create_likes_old so there is no conflict with file names within migrations.
2) Change the name of the table inside the migration from create_table :likes to create_table :likes_old.
3) Generate new create_likes migration that creates a table called Like.
Currently I'm running into the following issue when I run rake db:migrate:
PG::DuplicateTable: ERROR: relation "likes" already exists
This makes it seems like the Like table was never renamed to LikeOld. What's wrong and how do I fix this?

Old migrations don't run when you change their content, and you should not rename them. Even if they did run, changing the create_table :likes to create_table :old_likes cannot possibly change the name of an existing table. That isn't what create_table does. At best, re-running that migration will now cause a new table to be created called old_likes, with no content, while leaving your old likes table unaffected. In actuality re-running that migration will simply fail, as it will attempt to undo the migration first, dropping the table old_likes which does not yet exist.
What you need to do is create a new migration called rename_likes_to_old_likes, which actually renames the table, using rename_table. Then, either:
delete the old migration entirely, so you can introduce a new migration with the same name
OR
create a new migration with a unique name such as create_new_likes_table or the like, and introduce your new table there.

Related

rails db:migrate not working

I had a controller that I misnamed and ended up removing it from the command line via rails destroy controller. I ended up recreating everything but every time I have run rails db:migrate I get the following error.
SQLite3::SQLException: table "advertisements" already exists: CREATE TABLE "advertisements" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "title" varchar, "copy" text, "price" integer, "created_at" datetime NOT NULL, "updated_at" datetime NOT NULL)
I'm at a loss in terms of what to do, I've been dealing with this one tiny mistake for the last two days and everytime I seem to get on the right track one, yep. If anyone can tell me why this error is happening and what I can do I would greatly appreciate it. Thanks in advance
Take a look at the db/migrate directory. Could it bee that you have two xxxxx_create_advertisements.rb files (where xxxxxx is a bunch of numbers, more precisely the timestamp for when this migration was created)?
If so, then remove one of them (if both have the same column definitions then it does not matter which one - unless you have foreign key dependencies).
And then try again.
The issue is because your 'destroy_controller' command somehow didn't delete the table 'advertisements'. There are 2 solutions :
If you do not have any changes in the table then you don't need to recreate the table. Just comment out the migration in which you are creating table advertisements table.
If you really want to re-create the table then add drop_table :advertisements in your migration before creating the table, this will delete your existing table and then continue with the migration.
You can find the migration in db/migrate/ folder with name like 'xxxxx_create_advertisements.rb'
You should add a migration that drops the advertisements table.

Will I make many or just one migration for a model?

Are migrations instructions that change a model? Will I make several migrations, or will there be one migration per table? For instance, let's say if I want to change "username" to "admin_username". Does this call for a migration? Then, let's say I decide I want to add "age". Do I have to make a new migration, or do I just add it to the aforementioned migration?
You can make the quantity of migrations you want. My advise, try to plan your project and see how your model needs to be in order to avoid migrations. The migration feature is a great way to add, delete, rename fields to your model among other things.
If you need to change the username of the model User then you run rails g migration renameUsernameInUsers and add the necessary code to change the name of that field in you recently created migration file.
If some minutes later you realise that you also need to add a field you can make a new migration with your needs and add a field or add multiple fields in just one migration.
Remember, in order to apply your migrations you need to run rake db:migrate. If you modify the code in your migration files already migrated with this command, it will not have effect in your model. You will need a new migration.
In conclusion, you can(will) make many migrations for a model.
To go further on this topic you can see this official page for migrations in rails.

Rails - updating a model data members

I wanted to know what is the right way to update a Model.
for example, suppose I want to change a name of a data member from likes to numOfLikes.
this variable appears in the Model itself, but also in schema.rb and in db\migrate\XXX.rb
I changed manually those files and got this error:
undefined method `numOfLikes' for # Topic:0x3442d88
So, what is the right way doing it? (I am also asking about deleting a data member or adding one)
The best way is to run a migration to rename the column, which will update the schema.rb file.
You shouldn't be editing schema.rb directly, and the migration file only runs commands on the database, not affect the model if changed once run.
Schema.rb represents the state of the database schema, it doesn't control or change it by changing the contents of the file alone.
A new migration that contains:
rename_column :table_name, :likes, :numOfLikes
This will rename the column in the database, and will dump the database schema into schema.rb with the new attribute name.

How to create a migration deleting a table in ruby on rails

I want to delete a table in my rails app but i cannot use rollback, because i created this table a long time ago, and i have a LOT of other tables created since that one.
How is supposed to be named a drop table migration file and is there a way to genrate it with rails generate?
Create one more migration to drop the table. The class should have the method
def self.up
drop_table :table_name
end
Be careful as you will not be able to rollback to get all the data you will lose while dropping the table.

Rails: Best way to make changes to a production database

I need to make changes to an in-use production database. Just adding a few columns. I've made the changes to the dev database with migrations. What is the best way to update the production database while preserving the existing data and not disrupting operation too much?
It's MYSQL and I will be needing to add data to the columns as well for already existing records. One column can have a default value (it's boolean) but the other is a timestamp and should have an arbitrary backdated value. The row counts are not huge.
So if I use migrations how do I add data and how do I get it to just do the two (or three - I add data -latest migrations on the production db when it wasn't initially built via migrations (I believe they used the schema instead)?
I always follow this procedure:
Dump prod database with mysqldump command
Populate dev / test database with dump using mysql command
Run migrations in dev / test
Check migration worked
Dump prod database with mysqldump command (as it may have changed) keeping backup on server
Run migrations on prod (using capristano)
Test migration has worked on prod
Drink beer (while watching error logs)
It sounds like you're in a state where the production db schema doesn't exactly match what you're using in dev (although it's not totally clear). I would draw a line in the sand, and get that prod db in a better state. Essentially what you want to do is make sure that the prod db has a "schema_info" table that lists any migrations that you >don't< ever want to run in production. Then you can add migrations to your hearts content and they'll work against the production db.
Once you've done that you can write migrations that add schema changes or add data, but one thing you need to be really careful about is that if you add data using a migration, you must define the model within the migration itself, like this:
class AddSomeColumnsToUserTable < ActiveRecord::Migration
class User < ActiveRecord::Base; end
def self.up
add_column :users, :super_cool, :boolean, :default => :false
u = User.find_by_login('cameron')
u.super_cool = true
u.save
end
def self.down
remove_column :users, :super_cool
end
end
The reason for this is that in the future, you might remove the model altogether, during some refactoring or other. If you don't define the user class on line "User.find_by_login..." the migration will throw an exception which is a big pain.
Is there a reason you are not using the same migrations you used in your dev environment?
Adding a column with add_column in a migration should be non-destructive: it will generate a "ALTER TABLE" statement. If you know what you're going to put into the columns once created, you can fill in the values within the migration (you may choose a less time-consuming alternative if the row counts are large).
Removing or altering the definition of a column is, I think, platform-dependent: some will allow deletion of a column in place, others will perform a rename-create-select-drop sequence of commands.
To get more specific, we need more information: what kind of migration are you looking at, what platform are you running on, do you need to set values as part of the migration? Stuff like that would help a lot - just edit the question, which will push it back up the list.

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