Add unique constraint in users_groups (Issue benedmunds#454)

Issue benedmunds#454; tested with MSSQL, MySQL & PostgreSQL. MongoDB unaffected.

Author: daverogers

sql/ion_auth.mssql.sql

@@ -36,6 +36,7 @@ CREATE TABLE users_groups (
     user_id int NOT NULL,
 	group_id int NOT NULL,
   PRIMARY KEY(id),
+  CONSTRAINT uc_users_groups UNIQUE (user_id, group_id),
   CONSTRAINT users_groups_check_id CHECK(id >= 0),
   CONSTRAINT users_groups_check_group_id CHECK(group_id >= 0),
   CONSTRAINT users_groups_check_user_id CHECK(user_id >= 0)

sql/ion_auth.postgre.sql

@@ -36,6 +36,7 @@ CREATE TABLE "users_groups" (
     "user_id" integer NOT NULL,
     "group_id" integer NOT NULL,
   PRIMARY KEY("id"),
+  CONSTRAINT "uc_users_groups" UNIQUE (user_id, group_id),
   CONSTRAINT "users_groups_check_id" CHECK(id >= 0),
   CONSTRAINT "users_groups_check_user_id" CHECK(user_id >= 0),
   CONSTRAINT "users_groups_check_group_id" CHECK(group_id >= 0)

sql/ion_auth.sql

@@ -70,6 +70,7 @@ CREATE TABLE `users_groups` (
   PRIMARY KEY (`id`),
   KEY `fk_users_groups_users1_idx` (`user_id`),
   KEY `fk_users_groups_groups1_idx` (`group_id`),
+  CONSTRAINT `uc_users_groups` UNIQUE (`user_id`, `group_id`),
   CONSTRAINT `fk_users_groups_users1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE NO ACTION,
   CONSTRAINT `fk_users_groups_groups1` FOREIGN KEY (`group_id`) REFERENCES `groups` (`id`) ON DELETE CASCADE ON UPDATE NO ACTION
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
@@ -91,4 +92,4 @@ CREATE TABLE `login_attempts` (
   `login` varchar(100) NOT NULL,
   `time` int(11) unsigned DEFAULT NULL,
   PRIMARY KEY (`id`)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;