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

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

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+# 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.