Add Customize Template For New Schema Migration as a Rails TIL

rails/customize-template-for-new-schema-migration.md

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+# Customize Template For New Schema Migration
+
+Rails has a set of generator functionality that we can use to scaffold entire
+slices of an app all the way down to generating a single migration file.
+
+```bash
+$ rails generate migration MakeUserStatusColumnNotNull
+```
+
+When we run a migration generator command like that, Rails reaches for the
+[baked-in migration
+template](https://github.com/rails/rails/blob/92be9af152f721588b7414119c931ea92930947b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb.tt)
+and creates a new migration file based on the given name and any other local
+variables that get set internally.
+
+That's the standard behavior. However, we can override the migration template
+by defining our own template in `lib/templates/migration.rb.tt` of our Rails
+app. We'll need to follow the basic structure, but then we can alter it to our
+needs.
+
+For instance, I typically like to use the `#up` and `#down` methods and write
+raw SQL for my migrations. To help with that this template provides a good
+starting point.
+
+```ruby
+class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
+  def up
+    execute <<~SQL
+    SQL
+  end
+
+  def down
+    execute <<~SQL
+    SQL
+  end
+end
+```
+
+We can see in
+[`migration_generator.rb`](https://github.com/rails/rails/blob/92be9af152f721588b7414119c931ea92930947b/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb#L26-L43)
+how locals get set and what template gets chosen.