I don`t really understand what the difference is between a schema and a migration. Is a schema created by running a migration?
Rails schema.rb file represents the current state of the database.
It is generated and update when you run migrations with
rake db:migrate
Schema.rb is used when you do rake db:test:prepare or rake db:schema:dump, rake db:schema:load
Migrations are a convenient way for you to alter your database in a structured and organized manner. Active Record Migrations
Related
I initially ran a migration when I created the database and everything worked fine. Then I went and deleted the table in postgresql manually. Now when I run rake db:migrate, it runs but doesn't create the table.
It depends on what your migrations do. rake db:migrate can create or change tables, depending on your definitions. To setup the database and the tables in the beginning you can use rake db:schema:load or rake db:setup. rake -T gives an overview over all available rake tasks.
Database migrations are stored in the table schema_migrations which has one column version. As long as the version of the migration in question can be found here, the migration is not executed again.
I'm currently following the ruby on rails tutorial: http://guides.rubyonrails.org/getting_started.html.
I am trying to save data into the database. However, when I run: rails server I get the following error:
Migrations are pending. To resolve this issue, run: bin/rake db:migrate RAILS_ENV=development
I've looked at the other articles and when I run:
bin/rake db:migrate
I get a rake aborted!
After running:
rake db:abort_if_pending_migrations....
I see that:
You have 1 pending migration:
20150805200129 CreateDatabases
SQLite3::SQLException: table "databases" already exists:
and it just tells me to run rake db:migrate to start again.
It seems that it already exists. Is there a way to cancel the pending migration?
Sometimes, even dropping a local development database is not a good idea.
There are better ways to delete/destroy a specific migration in your Rails application.
You could use rails d migration command to destroy a particular migration:
rails d migration MigrationName
To undo the changes corresponding to a particular migration, you can use db:migrate:down method like this:
rake db:migrate:down VERSION=XXX
Sometimes, things could get more messy and in those situation another handy thing is to take a look at the schema_migrations table in your database which has all the migrations with their version saved in it.
You can delete a particular migration from this table like this:
delete from schema_migrations WHERE version = VERSION;
if you don't want that migration to be present anymore.
Your migration may have failed midway (so it created the table, but didn't finish).
You are just using development environment, so it's okay to just drop the database and rebuild it from scratch:
rake db:drop # THIS WILL DELETE YOUR DATABASE
rake db:create
rake db:migrate
If you are like me and maintain your database structure outside of Rails, you can just delete the migration file from db/migration. I got the error in the OP's question when I used the rails generate command to create a model class, forgetting that it also creates a migration file.
Do not use this method if you rely on Rails to maintain your database structure!
I keep my Rails structure file up to date by building it from the database using:
bundle exec rake db:structure:dump
I do not encourage to drop the database and start from the beginning especially when you already have the data inside the database.
My approach to this will be migrate first, then rollback. After that you can safely delete the migration file. So the procedure is as following.
rails db:migrate
rails db rollback
rm db/migrate/your_last_migration_file.rb
You can recreate database and run all migrations in your development environment with such command
rails db:migrate:reset
If you want to revert the wrong migrations, You can drop the whole db using this:
rake db:drop
Then remove the migrations file manually(This wont corrupt the db when you recreate as the Schema migrations would be dropped as well).
Then run
rake db:migrate
And if there is data to be seeded, then run this as well
rake db:setup
Is there a quick rake db:rollback command for all of the migrations?
Rolling back all migrations
To rollback all migrations the best solution is the one #Claudio Floreani proposed:
rake db:migrate VERSION=0
This will rollback every migration. You can read why this is the best approach in his answer. Then, run all migrations again with
rake db:migrate
Resetting the database
Reset
rake db:migrate:reset #runs db:drop db:create db:migrate
This method drops the database and runs the migrations again.
Loading the last schema
rake db:reset
This method will drop the database and load the data from the last schema.
You can see more information in this post: Difference between rake db:migrate db:reset and db:schema:load
Thanks to #Claudio Floreani and all the users who commented to improve the answer.
If you really want to rollback all of the migrations, and not just take the database to a pristine state or to the last schema, you have to run:
rake db:migrate VERSION=0
This will actually rollback all the way down every migration and ensure that every migration is reversible.
If you now issue
rake db:migrate:status
you will see that all the migrations are still there but they are in a 'down' (not applied) state.
Other commands that imply a rake db:reset or rake db:drop (such as in the answers by #Orlando or #Alex Falke) won't do any rollback at all: that is, they won't ensure that every migration is reversible.
Moreover, rake db:drop cannot be run while the database is being accessed by other users, while rollbacks can be performed live (even if this is generally not recommended). And last but not least, simply dropping and recreating the database will also delete the schema migrations table: if someone runs rake db:migrate:status after the database has been dropped, he will be replied with "Schema migrations table does not exist yet" and will have no clues about which migrations can be applied (unless he knows it yet or can list them).
just use rake db:reset, that will drop your database (same as undoing all migrations) and reset to the last schema.
UPDATE: a more correct approach will be using rake db:migrate:reset. That will drop the database, create it again and run all the migrations, instead of resetting to the latest schema.
If a permission problem raises (like it happened to me), maybe you could try dropping all database tables like I did with rubymine (just open database tool window, select all tables and right-click -> drop), it should be similar with other IDEs. Some tables like sqlite_master and sqlite_sequence were conveniently ignored in the drop.
This allowed me to do
rails db:migrate
and everything worked fine. Of course you loose all data!
Due to some deployment issues I stopped tracking schema.rb in git. Somehow I have stuffed this up and somewhere along the way my schema.rb file has disappeared.
Is there a way of regenerating schema.rb from the database or from the migrations? I would prefer not to lose the existing data.
If you run a rake -T it will list all possible rake tasks for your Rails project. One of them is db:schema:dump which will recreate the schema.rb for the Rails app from the database.
bundle exec rake db:schema:dump
Careful,
rake db:schema:dump
will dump the current DB schema FROM the DB. This means that if you made any changes to your migrations, they will NOT be reflected in schema.rb file which is not what you want IMO.
If you want to re-create the schema from the migrations, do the following:
rake db:drop # ERASES THE DATABASE !!!!
rake db:create
rake db:migrate
RAILS 5 Way:
rails db:schema:dump
or if you Encounter Gem::LoadError then:
bundle exec rails db:schema:dump
Note:
in rails 5 it is recommended that task are generated/executed by using rails instead of rake, this is just to remember, rails generated task are of extension .rake see in lib/tasks/myTask.rake. which means these task can also be executed by prepending rake.
rake db:schema:dump
I think this is still valid in Rails 3 - it regenerates the schema.rb from the database.
Directly from the schema.rb file itself:
If you need to create the application database on another
system, you should be using db:schema:load, not running all the migrations
from scratch. The latter is a flawed and unsustainable approach (the more migrations
you'll amass, the slower it'll run and the greater likelihood for issues).
So do NOT do the suggestion of rake db:migrate, which was suggested in the - at the time of this writing - lowest rated answer.
If you regenerate schema.rb locally, you should be alright. It simply holds a representation of the structure of your database tables. The data itself is not contained in this file.
To regenerate your schema.rb file, run:
bundle exec rake db:schema:dump
Then simply commit the new schema.rb file and you should be in good shape!
I also had a similar problem where my old schema was not refreshing even if I deleted migration.
So, what I did was dropping all existing tables in the database and migrating them again. Then running "db:schema:load" command gave me a fresh schema.rb.
drop table my_table_name // deleted them individually
rake db:migrate
rake db:schema:dump // re-created a new schema
I tried to run:
rake db:migrate VERSION=0
It reverts all migrations except the last one.
Then I tried to run:
rake db:migrate:down VERSION=<timestamp_of_last_migration>
but it didn't revert either. Why ?
Is there a command that runs all down methods at once ?
If your database only related to this project, and you are trying to undo everything in your migrations, I'd simply drop the database, and then run rake db:create.
Then you have an empty database ready to go.
Or is there another reason you're trying to run the down scripts?
You could check this list.
Maybe this could help you
rake db:create[:all]: If :all not specified then create the database defined in config/database.yml for the current RAILS_ENV. If :all is specified then create all of the databases defined in config/database.yml.
rake db:fixtures:load: Load fixtures into the current environment's database. Load specific fixtures using FIXTURES=x,y
rake db:migrate [VERSION=n]: Migrate the database through scripts in db/migrate. Target specific version with VERSION=n
rake db:migrate:redo [STEP=n]: (2.0.2) Revert the database by rolling back "STEP" number of VERSIONS and re-applying migrations.
rake db:migrate:reset: (2.0.2) Drop the database, create it and then re-apply all migrations. The considerations outlined in the note to rake db:create apply.
rake db:reset: Drop and re-create database using db/schema.rb. The considerations outlined in the note to rake db:create apply.
rake db:rollback [STEP=N]: (2.0.2) Revert migration 1 or n STEPs back.
rake db:schema:dump: Create a db/schema.rb file that can be portably used against any DB supported by AR
rake db:schema:load: Load a schema.rb file into the database
rake db:sessions:clear: Clear the sessions table
rake db:sessions:create: Creates a sessions table for use with CGI::Session::ActiveRecordStore
rake db:structure:dump: Dump the database structure to a SQL file
rake db:test:clone: Recreate the test database from the current environment's database schema
rake db:test:clone_structure: Recreate the test databases from the development structure
rake db:test:prepare: Prepare the test database and load the schema
rake db:test:purge: Empty the test database
there is another way:
rake db:rollback STEP=100
this will revert last 100 migrations
taken from http://guides.rubyonrails.org/migrations.html#rolling-back
try:
rake db:migrate:down VERSION=<timestamp_of_first_migration>
this will run the self.down for your first migration, essentially wiping everything out. at least, it just did for me!