basic previous(key) functionality

lib/dao.js

@@ -179,6 +179,10 @@ module.exports = (function() {
     }.bind(this))
     }.bind(this))
   }
   }
 
 
+  DAO.prototype.previous = function(key) {
+    return this._previousDataValues[key]
+  }
+
   DAO.prototype._setInclude = function(key, value, options) {
   DAO.prototype._setInclude = function(key, value, options) {
     if (!Array.isArray(value)) value = [value]
     if (!Array.isArray(value)) value = [value]
     if (value[0] instanceof DAO) {
     if (value[0] instanceof DAO) {

test/dao/values.test.js

@@ -184,5 +184,21 @@ describe(Support.getTestDialectTeaser("DAO"), function () {
         })
         })
       })
       })
     })
     })
+
+    describe('previous', function () {
+      it('should return the previous value', function () {
+        var User = this.sequelize.define('User', {
+          name: {type: DataTypes.STRING}
+        })
+
+        var user = User.build({
+          name: 'Jan Meier'
+        })
+        user.set('name', 'Mick Hansen')
+        
+        expect(user.previous('name')).to.equal('Jan Meier')
+        expect(user.get('name')).to.equal('Mick Hansen')
+      })
+    })
   })
   })
 })
 })
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