Add PostgreSQL function "is_nickname"

db/migrate/20181129203927_initial_migration.rb

@@ -2,6 +2,16 @@
 
 class InitialMigration < ActiveRecord::Migration[6.0]
   def change
+    func :is_nickname, <<~SQL
+      (str text) RETURNS boolean IMMUTABLE LANGUAGE plpgsql AS
+      $$
+      BEGIN
+        RETURN length(str) BETWEEN 3 AND 36
+          AND str ~ '^[a-z][a-z0-9]*(_[a-z0-9]+)*$';
+      END;
+      $$;
+    SQL
+
     func :is_good_text, <<~SQL
       (str text) RETURNS boolean IMMUTABLE LANGUAGE plpgsql AS
       $$
@@ -238,9 +248,7 @@ class InitialMigration < ActiveRecord::Migration[6.0]
     SQL
 
     constraint :accounts, :nickname, <<~SQL
-      length(nickname) BETWEEN 3 AND 36
-      AND
-      nickname ~ '^[a-z][a-z0-9]*(_[a-z0-9]+)*$'
+      is_nickname(nickname)
     SQL
 
     constraint :accounts, :public_name, <<~SQL

db/structure.sql

@@ -95,6 +95,20 @@ END;
 $_$;
 
 
+--
+-- Name: is_nickname(text); Type: FUNCTION; Schema: public; Owner: -
+--
+
+CREATE FUNCTION public.is_nickname(str text) RETURNS boolean
+    LANGUAGE plpgsql IMMUTABLE
+    AS $_$
+BEGIN
+  RETURN length(str) BETWEEN 3 AND 36
+    AND str ~ '^[a-z][a-z0-9]*(_[a-z0-9]+)*$';
+END;
+$_$;
+
+
 SET default_tablespace = '';
 
 SET default_with_oids = false;
@@ -149,7 +163,7 @@ CREATE TABLE public.accounts (
     contacts_list_id bigint NOT NULL,
     CONSTRAINT biography CHECK (((biography IS NULL) OR public.is_good_limited_text(biography, 3, 10000))),
     CONSTRAINT guest_token CHECK (((guest_token)::text ~ '^[0-9a-f]{32}$'::text)),
-    CONSTRAINT nickname CHECK ((((length((nickname)::text) >= 3) AND (length((nickname)::text) <= 36)) AND ((nickname)::text ~ '^[a-z][a-z0-9]*(_[a-z0-9]+)*$'::text))),
+    CONSTRAINT nickname CHECK (public.is_nickname((nickname)::text)),
     CONSTRAINT public_name CHECK (((public_name IS NULL) OR public.is_good_limited_text((public_name)::text, 3, 255)))
 );