strong_migrations

# Strong Migrations

Catch unsafe migrations in development

&nbsp;&nbsp;✓&nbsp;&nbsp;Detects potentially dangerous operations<br />&nbsp;&nbsp;✓&nbsp;&nbsp;Prevents them from running by default<br />&nbsp;&nbsp;✓&nbsp;&nbsp;Provides instructions on safer ways to do what you want

Supports PostgreSQL, MySQL, and MariaDB

:tangerine: Battle-tested at [Instacart](https://www.instacart.com/opensource)

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## Installation

Add this line to your application’s Gemfile:

```ruby
gem "strong_migrations"
```

And run:

```sh
bundle install
rails generate strong_migrations:install
```

Strong Migrations sets a long statement timeout for migrations so you can set a [short statement timeout](#app-timeouts) for your application.

## How It Works

When you run a migration that’s potentially dangerous, you’ll see an error message like:

```txt
=== Dangerous operation detected #strong_migrations ===

Active Record caches attributes, which causes problems
when removing columns. Be sure to ignore the column:

class User < ApplicationRecord
  self.ignored_columns += ["name"]
end

Deploy the code, then wrap this step in a safety_assured { ... } block.

class RemoveColumn < ActiveRecord::Migration[8.1]
  def change
    safety_assured { remove_column :users, :name }
  end
end
```

An operation is classified as dangerous if it either:

- Blocks reads or writes for more than a few seconds (after a lock is acquired)
- Has a good chance of causing application errors

## Checks

Potentially dangerous operations:

- [removing a column](#removing-a-column)
- [changing the type of a column](#changing-the-type-of-a-column)
- [renaming a column](#renaming-a-column)
- [renaming a table](#renaming-a-table)
- [creating a table with the force option](#creating-a-table-with-the-force-option)
- [adding an auto-incrementing column](#adding-an-auto-incrementing-column)
- [adding a stored generated column](#adding-a-stored-generated-column)
- [adding a foreign key](#adding-a-foreign-key)
- [adding a check constraint](#adding-a-check-constraint)
- [executing SQL directly](#executing-SQL-directly)
- [backfilling data](#backfilling-data)

Postgres-specific checks:

- [adding an index non-concurrently](#adding-an-index-non-concurrently)
- [adding a reference](#adding-a-reference)
- [adding a unique constraint](#adding-a-unique-constraint)
- [adding an exclusion constraint](#adding-an-exclusion-constraint)
- [adding a json column](#adding-a-json-column)
- [adding a column with a volatile default value](#adding-a-column-with-a-volatile-default-value)
- [setting NOT NULL on an existing column](#setting-not-null-on-an-existing-column)
- [renaming an enum value](#renaming-an-enum-value)
- [renaming a schema](#renaming-a-schema)

MySQL and MariaDB-specific checks:

- [using the COPY algorithm](#using-the-copy-algorithm)
- [using shared or exclusiv