Add Make Remove Column Migration Reversible as a Rails TIL

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).
 
-_1226 TILs and counting..._
+_1227 TILs and counting..._
 
 ---
 
@@ -767,6 +767,7 @@ _1226 TILs and counting..._
 - [Log SQL Queries Executed By ActiveRecord](rails/log-sql-queries-executed-by-activerecord.md)
 - [Mark A Migration As Irreversible](rails/mark-a-migration-as-irreversible.md)
 - [Make ActionMailer Synchronous In Test](rails/make-action-mailer-synchronous-in-test.md)
+- [Make Remove Column Migration Reversible](rails/make-remove-column-migration-reversible.md)
 - [Manually Run A Migration From Rails Console](rails/manually-run-a-migration-from-rails-console.md)
 - [Mark For Destruction](rails/mark-for-destruction.md)
 - [Mask An ActiveRecord Attribute](rails/mask-an-activerecord-attribute.md)

rails/make-remove-column-migration-reversible.md

@@ -0,0 +1,32 @@
+# Make Remove Column Migration Reversible
+
+The Rails migration DSL includes a
+[`#remove_column`](https://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-remove_column)
+method for removing a column from a table.
+
+The two required values are the table name and the column name.
+
+```ruby
+class RemoveNotesColumnFromTodo < ActiveRecord::Migration[5.2]
+  def change
+    remove_column :todos, :notes
+  end
+end
+```
+
+This on it's own is not reversible though. If you were to run a `db:rollback`
+with this migration, Rails wouldn't know what type to use when re-adding the
+column.
+
+To make it reversible, you need to add in the column type as a third argument.
+
+```ruby
+    remove_column :todos, :notes, :string
+```
+
+Note: perhaps more importantly, this column isn't reversible in the sense that
+when you remove the column, you are throwing away all the data associated with
+that column. Be sure you're ready to part with this data before deploying a
+migration like this.
+
+[source](https://stackoverflow.com/questions/24520550/how-do-you-make-remove-column-reversible)