Update alterTable to actually drop constraints

lib/migration.js

@@ -129,6 +129,7 @@ function mixinMigration(MySQL, mysql) {
     var newFkNames = Object.keys(newFks).filter(function(name) {
       return !!m.settings.foreignKeys[name];
     });
+    var dropConstraintSql = [];
     var sql = [];
     var ai = {};
 
@@ -185,7 +186,7 @@ function mixinMigration(MySQL, mysql) {
         }
 
         if (needsToDrop) {
-          sql.push('DROP FOREIGN KEY ' + fk.fkName);
+          dropConstraintSql.push('DROP FOREIGN KEY ' + fk.fkName);
           removedFks.push(fk); //keep track that we removed these
         }
       });
@@ -342,13 +343,22 @@ function mixinMigration(MySQL, mysql) {
         });
     }
 
-    if (sql.length) {
-      var query = 'ALTER TABLE ' + self.tableEscaped(model) + ' ' +
-        sql.join(',\n');
+    if (sql.length || dropConstraintSql.length) {
+      var stmtList = [dropConstraintSql, sql]
+        .filter(function(stmts) { return stmts.length; })
+        .map(function(statements) {
+          return 'ALTER TABLE ' + self.tableEscaped(model) +
+            ' ' + statements.join(',\n');
+        });
+
       if (checkOnly) {
-        done(null, true, {statements: sql, query: query});
+        done(null, true, {statements: stmtList, query: stmtList.join(';')});
       } else {
-        this.execute(query, done);
+        async.eachSeries(stmtList, function(stmt, execDone) {
+          self.execute(stmt, execDone);
+        }, function(err) {
+          done(err);
+        });
       }
     } else {
       done();