allow multiple calls of on for index creation

lib/dialect/postgres.js

@@ -415,7 +415,7 @@ Postgres.prototype.visitIndexes = function(node) {
 };
 
 Postgres.prototype.visitCreateIndex = function(node) {
-  if (!node.options.columns) {
+  if (!node.options.columns || (node.options.columns.length === 0)) {
     throw new Error('No columns defined!');
   }
 

lib/node/createIndex.js

@@ -12,7 +12,7 @@ module.exports = Node.define({
     this.names     = [];
     this.columns   = [];
     this.valueSets = [];
-    this.options   = { indexName: indexName };
+    this.options   = { indexName: indexName, columns: [] };
   },
 
   unique: function() {
@@ -36,7 +36,8 @@ module.exports = Node.define({
   },
 
   on: function() {
-    this.options.columns = Array.prototype.slice.call(arguments);
+    var args = Array.prototype.slice.call(arguments);
+    this.options.columns = this.options.columns.concat(args);
     return this;
   },
 

test/dialects/indexes-tests.js

@@ -38,6 +38,13 @@ Harness.test({
   sqlite: "CREATE INDEX \"post_id_userId\" ON \"post\" (\"post\".\"userId\",\"post\".\"id\")"
 });
 
+Harness.test({
+  query:  post.indexes().create().on(post.userId).on(post.id),
+  pg:     "CREATE INDEX \"post_id_userId\" ON \"post\" (\"post\".\"userId\",\"post\".\"id\")",
+  mysql:  "CREATE INDEX `post_id_userId` ON `post` (`post`.`userId`,`post`.`id`)",
+  sqlite: "CREATE INDEX \"post_id_userId\" ON \"post\" (\"post\".\"userId\",\"post\".\"id\")"
+});
+
 Harness.test({
   query:  post.indexes().create(),
   pg:     { text: 'No columns defined!', throws: true },