Add Validate Column Data With Check Constraints 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).
 
-_1447 TILs and counting..._
+_1448 TILs and counting..._
 
 ---
 
@@ -986,6 +986,7 @@ _1447 TILs and counting..._
 - [Update Column Versus Update Attribute](rails/update-column-versus-update-attribute.md)
 - [Upgrading Your Manifest For Sprocket's 4](rails/upgrading-your-manifest-for-sprockets-4.md)
 - [Use IRB And Ruby Flags With Rails Console](rails/use-irb-and-ruby-flags-with-rails-console.md)
+- [Validate Column Data With Check Constraints](rails/validate-column-data-with-check-constraints.md)
 - [Verify And Read A Signed Cookie Value](rails/verify-and-read-a-signed-cookie-value.md)
 - [Where Am I In The Partial Iteration?](rails/where-am-i-in-the-partial-iteration.md)
 - [Why Redirect And Return In Controllers](rails/why-redirect-and-return-in-controllers.md)

rails/validate-column-data-with-check-constraints.md

@@ -0,0 +1,32 @@
+# Validate Column Data With Check Constraints
+
+A check constraint is a feature of database systems like PostgreSQL that allows
+you to enforce rules about the data that goes in a table's column. As of Rails
+6.1, ActiveRecord provides a way to add a check constraint via the DSL.
+
+In this example, we want to ensure that the value going into the
+reading_statuses.status column is one of four values. Nothing else besides
+these four values should be allowed.
+
+```ruby
+class AddReadingStatusTable < ActiveRecord::Migration[7.2]
+  def change
+    create_table :reading_statuses do |t|
+      t.references :user, null: false, foreign_key: true
+      t.references :book, null: false, foreign_key: true
+      t.string :status, null: false
+      t.timestamps
+    end
+
+    add_check_constraint
+      :reading_statuses,
+      "status in ('started', 'completed', 'abandoned', 'already_read')",
+      name: "reading_statuses_valid_status_check"
+  end
+end
+```
+
+The `#add_check_constraint` method takes the name of the table and a SQL clause
+that can evaluate to true or false for a given row. We can optionally include
+the name of the check constraint (e.g. {table_name}_{column_name}_check) like
+we've done above.