let the transaction set the initial transaction state

lib/dialects/abstract/query-generator.js

@@ -464,6 +464,10 @@ module.exports = (function() {
       throwMethodUndefined('setAutocommitQuery')
     },
 
+    setIsolationLevelQuery: function(value) {
+      throwMethodUndefined('setIsolationLevelQuery')
+    },
+
     /**
      * Returns a query that starts a transaction.
      *

lib/dialects/mysql/query-generator.js

@@ -337,6 +337,10 @@ module.exports = (function() {
       return "SET autocommit = " + (!!value ? 1 : 0) + ";"
     },
 
+    setIsolationLevelQuery: function(value) {
+      return "SET SESSION TRANSACTION ISOLATION LEVEL " + value + ";"
+    },
+
     /**
      * Returns a query that starts a transaction.
      *

lib/query-interface.js

@@ -808,6 +808,15 @@ module.exports = (function() {
     return this.queryAndEmit([sql, null, { transaction: transaction }], 'setAutocommit')
   }
 
+  QueryInterface.prototype.setIsolationLevel = function(transaction, value) {
+    if (!transaction || !(transaction instanceof Transaction)) {
+      throw new Error('Unable to set isolatipn level for a transaction without transaction object!')
+    }
+
+    var sql = this.QueryGenerator.setIsolationLevelQuery(value)
+    return this.queryAndEmit([sql, null, { transaction: transaction }], 'setIsolationLevel')
+  }
+
   QueryInterface.prototype.startTransaction = function(transaction, options) {
     if (!transaction || !(transaction instanceof Transaction)) {
       throw new Error('Unable to start a transaction without transaction object!')

lib/sequelize.js

@@ -393,28 +393,15 @@ module.exports = (function() {
       , transaction = new Transaction(this, options)
       , self        = this
 
-    var startTransaction = function() {
-      self
-        .getQueryInterface()
-        .startTransaction(transaction, {})
-        .success(function() {
-          callback(transaction)
-        })
-    }
-
-    var setAutocommit = function(value, cb) {
-      self
-        .getQueryInterface()
-        .setAutocommit(transaction, value)
-        .success(cb)
-    }
-
     Utils.tick(function() {
-      if (options.hasOwnProperty('autocommit')) {
-        setAutocommit(!!options.autocommit, startTransaction)
-      } else {
-        startTransaction()
-      }
+      transaction.prepareEnvironment(function() {
+        self
+          .getQueryInterface()
+          .startTransaction(transaction, {})
+          .success(function() {
+            callback(transaction)
+          })
+      })
     })
 
     return transaction

lib/transaction.js

@@ -3,12 +3,22 @@ var Utils = require('./utils')
 
 var Transaction = module.exports = function(sequelize, options) {
   this.sequelize = sequelize
-  this.options   = options || {}
   this.id        = Utils.generateUUID()
+  this.options   = Utils._.extend({
+    autocommit: true,
+    isolationLevel: Transaction.ISOLATION_LEVELS.REPEATABLE_READ
+  }, options || {})
 }
 
 util.inherits(Transaction, Utils.CustomEventEmitter)
 
+Transaction.ISOLATION_LEVELS = {
+  READ_UNCOMMITTED: "READ UNCOMMITTED",
+  READ_COMMITTED: "READ COMMITTED",
+  REPEATABLE_READ: "REPEATABLE READ",
+  SERIALIZABLE: "SERIALIZABLE"
+}
+
 Transaction.prototype.commit = function() {
   return this
     .sequelize
@@ -25,3 +35,29 @@ Transaction.prototype.rollback = function() {
     .rollbackTransaction(this, {})
     .proxy(this)
 }
+
+Transaction.prototype.prepareEnvironment = function(callback) {
+  var self  = this
+
+  this.setIsolationLevel(function() {
+    self.setAutocommit(function() {
+      callback()
+    })
+  })
+}
+
+Transaction.prototype.setAutocommit = function(callback) {
+  this
+    .sequelize
+    .getQueryInterface()
+    .setAutocommit(this, this.options.autocommit)
+    .success(callback)
+}
+
+Transaction.prototype.setIsolationLevel = function(callback) {
+  this
+    .sequelize
+    .getQueryInterface()
+    .setIsolationLevel(this, this.options.isolationLevel)
+    .success(callback)
+}