Add Remove A Database Column From A Table as a Rails TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1235 TILs and counting..._
+_1236 TILs and counting..._
 
 ---
 
@@ -788,6 +788,7 @@ _1235 TILs and counting..._
 - [Query A Single Value From The Database](rails/query-a-single-value-from-the-database.md)
 - [Read In Environment-Specific Config Values](rails/read-in-environment-specific-config-values.md)
 - [Read-Only Models](rails/read-only-models.md)
+- [Remove A Database Column From A Table](rails/remove-a-database-column-from-a-table.md)
 - [Remove The Default Value On A Column](rails/remove-the-default-value-on-a-column.md)
 - [Render An Alternative ActionMailer Template](rails/render-an-alternative-action-mailer-template.md)
 - [Render The Response Body In Controller Specs](rails/render-the-response-body-in-controller-specs.md)

rails/remove-a-database-column-from-a-table.md

@@ -0,0 +1,34 @@
+# Remove A Database Column From A Table
+
+The `ActiveRecord` migration DSL includes a method
+[`remove_column`](https://api.rubyonrails.org/v7.0.3.1/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-remove_column)
+that can be used to remove an existing column from a table.
+
+It can be used like so, to remove the `sign_in_count` column from the `users`
+table.
+
+```ruby
+def change
+  remove_column :users, :sign_in_count
+end
+```
+
+Though that will work fine, you'll run into an `IrreversibleMigration` error if
+you try to `rails db:rollback`. It usually a good bet to make migrations
+reversible when it is easy to do so.
+
+All we need in order to make this migration reversible is to add the column
+type.
+
+```ruby
+def change
+  remove_column :users, :sign_in_count, :integer
+end
+```
+
+Now you can rollback (or [migrate up-down-up](migrating-up-down-up.md)) no
+problem.
+
+Keep in mind that only the structural changes are reversible. When you remove
+the column, all of the data goes with it, and that cannot be undone with a
+simple rollback.