diff --git a/Readme.md b/Readme.md
index 3102c9ed..e239933c 100755
--- a/Readme.md
+++ b/Readme.md
@@ -586,7 +586,10 @@ See information in the [wiki](https://github.com/dresende/node-orm2/wiki/Model-V
 
 ## Associations
 
-An association is a relation between one or more tables.
+An association is a relation between one or more tables. This version of node-orm2 creates foreign keys with MySQL for hasOne associations.
+
+### foreign keys with hasOne
+`Animal.hasOne('owner', Person, { onDelete: 'cascade' })` (or **set null**, or **restrict**)
 
 ### hasOne
 
diff --git a/lib/Drivers/DDL/mysql.js b/lib/Drivers/DDL/mysql.js
index 1d521d4d..1f3d16c1 100755
--- a/lib/Drivers/DDL/mysql.js
+++ b/lib/Drivers/DDL/mysql.js
@@ -70,7 +70,7 @@ exports.sync = function (driver, opts, cb) {
 		"CREATE TABLE IF NOT EXISTS " + driver.query.escapeId(opts.table) +
 		" (" + definitions.join(", ") + ")"
 	);
-
+            
 	for (i = 0; i < opts.many_associations.length; i++) {
 		definitions = [];
 
@@ -116,6 +116,45 @@ exports.sync = function (driver, opts, cb) {
 	}
 };
 
+exports.syncConstraints = function (driver, opts, cb) {
+	var queries = [];
+	var k, i, pending;
+
+	for (i = 0; i < opts.one_associations.length; i++) {
+		if (opts.one_associations[i].extension) continue;
+		if (opts.one_associations[i].reversed) continue;
+		for (k in opts.one_associations[i].field) {
+                    queries.push(
+                            "ALTER TABLE " + driver.query.escapeId(opts.table) + " " +
+                            "DROP FOREIGN KEY fk_" + k + "_" + opts.one_associations[i].name
+                    );
+                    queries.push(
+                            "ALTER TABLE " + driver.query.escapeId(opts.table) + " " +
+                            "ADD CONSTRAINT fk_" + k + "_" + opts.one_associations[i].model.table + " FOREIGN KEY (" + driver.query.escapeId(k) + ") " +
+                            "REFERENCES " + opts.one_associations[i].name + "(`id`) " + 
+                            "ON UPDATE " + (opts.one_associations[i].onUpdate ? opts.one_associations[i].onUpdate.toUpperCase() : 'CASCADE') + " " + 
+                            "ON DELETE " + (opts.one_associations[i].onDelete ? opts.one_associations[i].onDelete.toUpperCase() : 'RESTRICT')
+                    );
+		}
+	}
+
+	for (i = 0; i < opts.many_associations.length; i++) {
+		// TODO: build this
+	}
+
+	pending = queries.length;
+        
+        if (!queries.length) return cb();
+        
+	for (i = 0; i < queries.length; i++) {
+		driver.execQuery(queries[i], function (err) {
+			if (--pending === 0) {
+				return cb(err && err.code === 'ER_CANT_CREATE_TABLE' ? null : err);
+			}
+		});
+	}
+};
+
 var colTypes = {
 	integer:  { 2: 'SMALLINT', 4: 'INTEGER', 8: 'BIGINT' },
 	floating: {                4: 'FLOAT',   8: 'DOUBLE' }
diff --git a/lib/Drivers/DML/mysql.js b/lib/Drivers/DML/mysql.js
index d73b5d08..971786ae 100755
--- a/lib/Drivers/DML/mysql.js
+++ b/lib/Drivers/DML/mysql.js
@@ -32,6 +32,10 @@ Driver.prototype.sync = function (opts, cb) {
 	return require("../DDL/mysql").sync(this, opts, cb);
 };
 
+Driver.prototype.syncConstraints = function (opts, cb) {
+	return require("../DDL/mysql").syncConstraints(this, opts, cb);
+};
+
 Driver.prototype.drop = function (opts, cb) {
 	return require("../DDL/mysql").drop(this, opts, cb);
 };
diff --git a/lib/Instance.js b/lib/Instance.js
index 68f624b8..248154e0 100755
--- a/lib/Instance.js
+++ b/lib/Instance.js
@@ -29,7 +29,7 @@ function Instance(Model, opts) {
 		var pending = [], errors = [], required;
 
 		Hook.wait(instance, opts.hooks.beforeValidation, function (err) {
-		    var k, i;
+		    var k, i, a;
 			if (err) {
 				return saveError(cb, err);
 			}
@@ -62,13 +62,15 @@ function Instance(Model, opts) {
 				required = false;
 
 				if (Model.properties[k]) {
-					required = Model.properties[k].required;
+					required = Model.properties[k].alwaysValidate || Model.properties[k].required;
 				} else {
 					for (i = 0; i < opts.one_associations.length; i++) {
-						if (opts.one_associations[i].field === k) {
-							required = opts.one_associations[i].required;
-							break;
-						}
+                                                for (a in opts.one_associations[i].field) {
+                                                    if (a === k) {
+                                                            required = opts.one_associations[i].alwaysValidate || opts.one_associations[i].required;
+                                                            break;
+                                                    }
+                                                }
 					}
 				}
 				if (!required && instance[k] == null) {
diff --git a/lib/Model.js b/lib/Model.js
index 188ba3a9..948c1d41 100644
--- a/lib/Model.js
+++ b/lib/Model.js
@@ -83,7 +83,7 @@ function Model(opts) {
 			autoFetchLimit : inst_opts.autoFetchLimit,
 			cascadeRemove  : inst_opts.cascadeRemove
 		};
-		var pending  = 2, create_err = null;
+		var pending  = 2;
 		var instance = new Instance(model, {
 			uid                    : inst_opts.uid, // singleton unique id
 			id                     : opts.id,
@@ -104,12 +104,9 @@ function Model(opts) {
 			association_properties : association_properties
 		});
 		instance.on("ready", function (err) {
-			if (--pending > 0) {
-				create_err = err;
-				return;
-			}
+			if (--pending > 0) return;
 			if (typeof cb === "function") {
-				return cb(err || create_err, instance);
+				return cb(err, instance);
 			}
 		});
 		if (model_fields !== null) {
@@ -123,12 +120,9 @@ function Model(opts) {
 			ManyAssociation.autoFetch(instance, many_associations, assoc_opts, function () {
 				ExtendAssociation.autoFetch(instance, extend_associations, assoc_opts, function () {
 					Hook.wait(instance, opts.hooks.afterAutoFetch, function (err) {
-						if (--pending > 0) {
-							create_err = err;
-							return;
-						}
+						if (--pending > 0) return;
 						if (typeof cb === "function") {
-							return cb(err || create_err, instance);
+							return cb(err, instance);
 						}
 					});
 				});
@@ -236,6 +230,33 @@ function Model(opts) {
 
 		return cb(ErrorCodes.generateError(ErrorCodes.NO_SUPPORT, "Driver does not support Model.sync()", { model: opts.table }));
 	};
+	model.syncConstraints = function (cb) {
+		if (arguments.length === 0) {
+			cb = function () {};
+		}
+		if (typeof opts.driver.syncConstraints === "function") {
+			try {
+				opts.driver.syncConstraints({
+					extension           : opts.extension,
+					id                  : opts.id,
+					table               : opts.table,
+					properties          : opts.properties,
+					allProperties       : allProperties,
+					indexes             : opts.indexes || [],
+					customTypes         : opts.db.customTypes,
+					one_associations    : one_associations,
+					many_associations   : many_associations,
+					extend_associations : extend_associations
+				}, cb);
+			} catch (e) {
+				return cb(e);
+			}
+
+			return this;
+		}
+
+		return cb(ErrorCodes.generateError(ErrorCodes.NO_SUPPORT, "Driver does not support Model.syncConstraint()", { model: opts.table }));
+	};
 
 	model.get = function () {
 		var conditions = {};
diff --git a/lib/ORM.js b/lib/ORM.js
index 65df1023..7e102f40 100644
--- a/lib/ORM.js
+++ b/lib/ORM.js
@@ -283,11 +283,15 @@ ORM.prototype.load = function () {
 
 	return loadNext();
 };
-ORM.prototype.sync = function (cb) {
+ORM.prototype.sync = function (constraints, cb) {
 	var modelIds = Object.keys(this.models);
 	var syncNext = function () {
 		if (modelIds.length === 0) {
-			return cb();
+                        if (constraints) {
+                                return this.syncConstraints(cb);
+                        } else {
+                                return cb();
+                        }
 		}
 
 		var modelId = modelIds.shift();
@@ -303,10 +307,42 @@ ORM.prototype.sync = function (cb) {
 		});
 	}.bind(this);
 
-	if (arguments.length === 0) {
-		cb = function () {};
-	}
+        if (typeof constraints === 'function') {
+                cb = constraints;
+                constraints = false;
+        }
+        if (!cb) {
+                cb = function () {};
+        }
+        
+	syncNext();
+
+	return this;
+};
+ORM.prototype.syncConstraints = function (cb) {
+	var modelIds = Object.keys(this.models);
+	var syncNext = function () {
+		if (modelIds.length === 0) {
+			return cb();
+		}
+
+		var modelId = modelIds.shift();
+
+		this.models[modelId].syncConstraints(function (err) {
+			if (err) {
+				err.model = modelId;
+
+				return cb(err);
+			}
+
+			return syncNext();
+		});
+	}.bind(this);
 
+        if (arguments.length === 0) {
+                cb = function () {};
+        }
+        
 	syncNext();
 
 	return this;