Add Ensure A Rake Task Cannot Write Data 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).
 
-_1281 TILs and counting..._
+_1282 TILs and counting..._
 
 ---
 
@@ -789,6 +789,7 @@ _1281 TILs and counting..._
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
 - [Different Ways To Add A Foreign Key Reference](rails/different-ways-to-add-a-foreign-key-reference.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
+- [Ensure A Rake Task Cannot Write Data](rails/ensure-a-rake-task-cannot-write-data.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Find Or Create A Record With FactoryBot](rails/find-or-create-a-record-with-factory-bot.md)
 - [Find Records With Multiple Associated Records](rails/find-records-with-multiple-associated-records.md)

rails/ensure-a-rake-task-cannot-write-data.md

@@ -0,0 +1,41 @@
+# Ensure A Rake Task Cannot Write Data
+
+Let's say we are writing a substantially complex rake task for aggregating,
+arranging, and exporting data from our app's database. The idea is to run this
+rake task against production.
+
+There is no part of this rake task that needs to write data. It should act as a
+read-only script. And in fact, we can ensure it isn't able to write any data.
+We can do that by putting `ApplicationRecord` in read-only mode.
+
+```ruby
+task big_export_rake_task: :environment do
+  class ApplicationRecord
+    def readonly?
+      true
+    end
+  end
+
+  # a bunch of logic ...
+  puts 'this gets executed'
+
+  # Call method that inadvertently writes data
+  User.update(email: 'readonly@email.com')
+
+  # more logic ...
+  puts 'this does not get executed'
+end
+```
+
+Because we have made all of `ApplicationRecord` read-only, when we run this
+task, it is immediately going to rollback the changes that were attempted and
+then raise an error which aborts the rest of the rake task.
+
+We'll see some messaging like this:
+
+```
+rake aborted!
+ActiveRecord::ReadOnlyRecord: User is marked as readonly
+```
+
+h/t [Dillon Hafer](https://dillonhafer.com/)