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# How to rails migration works?

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Rails migrations are a powerful feature that allows you to evolve your database schema over time in a consistent and easy way. They are used to create, modify, and manage your database tables and columns. Here’s a step-by-step overview of how Rails migrations work:

1. Creating a Migration

You can generate a new migration using the Rails generator command:

rails generate migration AddFieldToTable field_name:data_type
For example, to add a name column to the users table:

rails generate migration AddNameToUsers name:string
2. Writing the Migration

The generated migration file will be located in the db/migrate directory. It will look something like this:

class AddNameToUsers < ActiveRecord::Migration[6.0]
  def change
    add_column :users, :name, :string
  end
end
You can define the changes you want to make in the change method. Rails provides several methods to manipulate the database schema, such as create_table, add_column, remove_column, rename_column, and more.

3. Running the Migration

To apply the migration and update your database schema, run:

rails db:migrate
This command will execute the change method in your migration file and make the necessary changes to the database.

4. Rolling Back a Migration

If you need to undo a migration, you can roll it back using:

rails db:rollback
This will reverse the last migration that was run. You can also specify the number of steps to roll back:

rails db:rollback STEP=2
5. Schema Versioning

Rails keeps track of which migrations have been applied to the database using a special table called schema_migrations. This ensures that each migration is only run once.

6. Schema File

After running migrations, Rails updates the db/schema.rb file to reflect the current state of the database schema. This file can be used to recreate the database schema without running all the migrations.

Example Migration

Here’s an example of a more complex migration that creates a products table with various columns:

class CreateProducts < ActiveRecord::Migration[6.0]
  def change
    create_table :products do |t|
      t.string :name
      t.text :description
      t.decimal :price, precision: 8, scale: 2
      t.integer :stock

      t.timestamps
    end
  end
end
Best Practices
  • Version Control: Always commit your migration files to version control.
  • Descriptive Names: Use descriptive names for your migrations to make it clear what changes are being made.
  • Test Migrations: Run your migrations in a development environment before applying them to production.
Rails migrations help you manage your database schema changes in a systematic and version-controlled manner, making it easier to collaborate with your team and maintain your application over time.