Strong_migrations Gem Conflicts: Code Examples & Resolutions

Navigating database migrations can be tricky, especially when dealing with existing applications and evolving schemas. The strong_migrations gem is a powerful tool for ensuring the safety and efficiency of these migrations in Ruby on Rails applications. However, conflicts can arise, particularly when adding NOT NULL constraints to columns. This article delves into a common conflict encountered with strong_migrations, provides a detailed code example, and outlines the best practices for resolving such issues, ensuring smooth and safe database schema changes.

Understanding the strong_migrations Gem

The strong_migrations gem is designed to prevent downtime and data loss during database migrations. It achieves this by enforcing a set of safety checks that flag potentially dangerous operations, such as adding NOT NULL constraints to existing columns or large table alterations. These operations can lock tables, block reads and writes, and lead to significant performance degradation in production environments. By identifying these risky operations early on, strong_migrations encourages developers to adopt safer migration strategies.

Why strong_migrations Flags NOT NULL Constraints

Adding a NOT NULL constraint to an existing column seems straightforward, but it can be surprisingly disruptive. When a database system adds a NOT NULL constraint, it must scan the entire table to ensure that all existing rows have a value for the column. This process can take a considerable amount of time, especially for large tables, during which the table might be locked, preventing reads and writes. Such locks can lead to application downtime and a poor user experience. strong_migrations flags this operation as dangerous to promote a more gradual and safer approach.

The Recommended Approach: Check Constraints

Instead of directly adding a NOT NULL constraint, strong_migrations advocates for using check constraints. A check constraint allows you to define a condition that must be true for all rows in a table. In this case, you can add a check constraint that verifies that the column is not null. However, unlike a NOT NULL constraint, adding a check constraint does not immediately validate existing rows. This allows you to add the constraint without locking the table.

Code Example: The Conflict and the Solution

Let's consider a scenario where you want to add a NOT NULL constraint to an email column in a users table. A naive approach might involve the following migration:

class AddNotNullToUsersEmail < ActiveRecord::Migration[7.0]
  def change
    change_column_null :users, :email, false
  end
end

When you run this migration with strong_migrations enabled, you'll encounter the following error:

StandardError: An error has occurred, this and all later migrations canceled: (StandardError)

=== Dangerous operation detected #strong_migrations ===

Setting NOT NULL on an existing column blocks reads and writes while every row is checked.
Instead, add a check constraint and validate it in a separate migration.

This error message clearly indicates that strong_migrations has detected a potentially dangerous operation. It suggests using a check constraint instead. Here’s how you can implement the recommended approach:

Step 1: Add a Check Constraint

The first step is to add a check constraint to the users table. This constraint will enforce the NOT NULL condition on the email column for new and updated rows. Create a new migration with the following content:

class AddEmailNotNullCheckConstraintToUsers < ActiveRecord::Migration[7.0]
  def change
    add_check_constraint :users, 'email IS NOT NULL', name: 'email_not_null'
  end
end

This migration adds a check constraint named email_not_null to the users table, ensuring that the email column cannot be null for new or updated records. The beauty of this approach is that it doesn't lock the table or disrupt existing operations.

Step 2: Backfill Existing Data (If Necessary)

If your table contains rows with null values in the email column, you'll need to backfill these values before enforcing the NOT NULL constraint fully. This can be done using a separate migration or a background job, depending on the size of your table and the acceptable downtime. A simple migration might look like this:

class BackfillUsersEmail < ActiveRecord::Migration[7.0]
  def up
    User.where(email: nil).update_all(email: 'default@example.com')
  end

  def down
    # Revert the backfill if needed
    User.where(email: 'default@example.com').update_all(email: nil)
  end
end

Important: Choose a suitable default value for your application context. The example above uses default@example.com, but you might need a different value or a more sophisticated approach to populate the missing email addresses.

Step 3: Validate the Check Constraint

After adding the check constraint and backfilling the data, you need to validate the constraint. This step ensures that all existing rows comply with the NOT NULL condition. strong_migrations provides a helper method for this purpose.

Create another migration:

class ValidateEmailNotNullCheckConstraintOnUsers < ActiveRecord::Migration[7.0]
  def change
    validate_check_constraint :users, 'email_not_null'
  end
end

This migration uses the validate_check_constraint method provided by strong_migrations. This method performs the validation in a safe and efficient manner, without locking the table for an extended period.

Step 4: Enforce the NOT NULL Constraint

Once the check constraint is validated, you can safely enforce the NOT NULL constraint by removing the check constraint and adding the NOT NULL constraint directly. This step finalizes the process and ensures that the database enforces the NOT NULL condition.

Create a final migration:

class EnforceNotNullOnUsersEmail < ActiveRecord::Migration[7.0]
  def change
    remove_check_constraint :users, name: 'email_not_null'
    change_column_null :users, :email, false
  end
end

This migration first removes the check constraint and then adds the NOT NULL constraint. By this point, you've ensured that all existing data complies with the constraint, making the operation safe.

Using safety_assured

In some cases, you might be confident that a migration is safe to run without the recommended precautions. strong_migrations provides a safety_assured method that allows you to bypass the safety checks. However, use this method with caution and only when you fully understand the implications of the operation.

For example, if you're working on a brand-new table with no data, adding a NOT NULL constraint might be safe. In such cases, you can use safety_assured:

class AddNotNullToUsersEmail < ActiveRecord::Migration[7.0]
  def change
    safety_assured {
      change_column_null :users, :email, false
    }
  end
end

Warning: Overusing safety_assured can defeat the purpose of strong_migrations. Always consider the potential risks and use it judiciously.

Key Takeaways

• The strong_migrations gem is essential for ensuring safe database migrations in Rails applications.
• Adding NOT NULL constraints to existing columns can be dangerous and lead to downtime.
• The recommended approach is to use check constraints and validate them in separate migrations.
• The safety_assured method should be used sparingly and only when the operation is known to be safe.
• Always follow the principles of gradual and safe database changes to minimize risks.

Best Practices for Database Migrations

Beyond the specific case of NOT NULL constraints, several best practices can help you manage database migrations effectively:

1. Keep Migrations Small and Focused: Each migration should address a single, logical change. This makes it easier to understand, review, and revert migrations if necessary.
2. Use Idempotent Migrations: Migrations should be idempotent, meaning they can be run multiple times without causing errors or unintended side effects. This is crucial for deployment and rollback scenarios.
3. Test Your Migrations: Always test your migrations in a development or staging environment before applying them to production. This helps identify potential issues early on.
4. Monitor Your Migrations: During and after running migrations in production, monitor your database performance to ensure that the changes are not causing any performance degradation.
5. Use a Migration Tool: Tools like strong_migrations can help you identify and prevent dangerous operations, ensuring the safety of your database.

Addressing Specific Concerns for Vets API Database Migrations

For teams working on the Vets API, adhering to these practices is particularly critical due to the scale and sensitivity of the data involved. The Vets API documentation emphasizes the importance of safe database migrations, and strong_migrations is a key tool in achieving this.

The documentation highlights the need for adding columns without default values carefully. This is because adding a column with a default value can also lock the table during the migration process. The recommended approach is to add the column without a default value, backfill the data in a separate migration, and then add the default value if needed.

Conclusion

Database migrations are a critical part of application development, and strong_migrations is an invaluable tool for ensuring their safety and efficiency. By understanding the potential risks associated with certain operations and following the recommended best practices, you can minimize downtime and data loss. This article has provided a detailed example of resolving a common conflict with strong_migrations when adding NOT NULL constraints, as well as broader guidance for managing database migrations effectively. By adopting these strategies, development teams can confidently evolve their database schemas while maintaining the stability and performance of their applications.

For further reading on safe database migrations and best practices, you can visit this external resource.