Add Add A Check Constraint To A Table as a rails til

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).         
 
-_860 TILs and counting..._
+_861 TILs and counting..._
 
 ---
 
@@ -487,6 +487,7 @@ _860 TILs and counting..._
 
 ### Rails
 
+- [Add A Check Constraint To A Table](rails/add-a-check-constraint-to-a-table.md)
 - [Add A Foreign Key Reference To A Table](rails/add-a-foreign-key-reference-to-a-table.md)
 - [Add React With Webpacker To A New Rails App](rails/add-react-with-webpacker-to-a-new-rails-app.md)
 - [Access Secrets In A Rails 5.2 App](rails/access-secrets-in-a-rails-5-2-app.md)

rails/add-a-check-constraint-to-a-table.md

@@ -0,0 +1,35 @@
+# Add A Check Constraint To A Table
+
+PostgreSQL allows you to enforce all kinds of rules about the value of a column
+or the relationship between two columns. These rules are defined with [_check
+constraints_](https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS).
+ActiveRecord's migration DSL does not provide a way for adding check
+constraints directly. They can be added by executing a SQL statement in a
+migration.
+
+```ruby
+class EnsurePageCountIsPositive < ActiveRecord::Migration[5.2]
+  def up
+    execute <<-SQL
+      alter table books
+        add constraint ensure_page_count_is_positive
+        check (page_count > 0);
+    SQL
+  end
+
+  def down
+    execute <<-SQL
+      alter table books
+        drop constraint ensure_page_count_is_positive;
+    SQL
+  end
+```
+
+This check constraint ensures that, anytime you add or update a row in the book
+column, the value of `page_count` column is always greater than `0`. This is a
+nice thing to enforce because it wouldn't make much sense for a book to have,
+say, `-10` pages.
+
+Note: these constraints will not appear in your `db/schema.rb` file. If you
+want to see what check constraints have been defined across your tables, you
+can crack open `psql` to investigate.