Add Ensure A Rake Task Cannot Write Data as a Rails TIL

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

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+# 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/)