Add constraints to `accounts`.`username`

db/migrate/20190201013649_add_lower_case_constraint_to_username_of_accounts.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+class AddLowerCaseConstraintToUsernameOfAccounts < ActiveRecord::Migration[6.0]
+  def change
+    reversible do |dir|
+      dir.up do
+        add_constraint :lower_case, 'username = lower(username)'
+        add_constraint :min_length, 'length(username) >= 3'
+        add_constraint :max_length, 'length(username) <= 36'
+      end
+
+      dir.down do
+        drop_constraint :lower_case
+        drop_constraint :min_length
+        drop_constraint :max_length
+      end
+    end
+  end
+
+private
+
+  def add_constraint(name, constraint)
+    execute 'ALTER TABLE accounts '                           \
+            "ADD CONSTRAINT accounts_username_#{name}_check " \
+            "CHECK (#{constraint})"
+  end
+
+  def drop_constraint(name)
+    execute 'ALTER TABLE accounts '                           \
+            "DROP CONSTRAINT accounts_username_#{name}_check"
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_02_01_012021) do
+ActiveRecord::Schema.define(version: 2019_02_01_013649) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"