Extending Entities

While Breeze can create an entity based exclusively on its metadata description, you can add additional properties and behaviors as needed to support client-side requirements. For example, you may want to add a method that is useful when presenting data to the user. This topic explains how to extend the entity definition with members that are not defined in metadata.

Most of the topic examples are based on tests in the entityExtensionTests module of DocCode.

Brute force extension

Breeze entities are created by calling the EntityManager.createEntity method or they are materialized as a result of an EntityQuery or EntityManager.importEntities. Let’s stick with createEntity for the moment.

var cust = manager.createEntity('Customer');

Internally that method first acquires an EntityType information object from a MetadataStore and then calls the type’s createEntity method [1], e.g.,

// assume that manager's metadata was previously fetched from the server
var store = manager.metadataStore;
var customerType = store.getEntityType('Customer');
var cust = customerType.createEntity();

Now suppose that, in our client application, we want every Customer to have an isBeingEdited property. This is a client-side-only property; it is completely unknown on the server and it’s not in the metadata we got from the server.

Because this is JavaScript, we can extend a customer object simply by patching it with the new property:

// AngularJS or Aurelia:
cust.isBeingEdited = false;

// Knockout observable (because we'll bind to it):
cust.isBeingEdited = ko.observable(false);

That’s not so bad. We’ll probably do this a few more times so we encapsulate all of this plumbing into a customerFactory method.

Unfortunately, that only covers the entity creation use case. Breeze also materializes entities from query results. Consider this query:

var query = breeze.EntityQuery.for('Customers')

When executed, the query will return Customer entities that may or may not already be in the client cache. We can’t use the customerFactory - it’s only good for creating new entities. So we post-process the materialized Customer entities in the query callback:

  .then(function (data) {
    var customers = data.results;
    customers.forEach(function (c) {
        if (c.isBeingEdited === undefined) { 
            c.isBeingEdited = ko.observable(false); }

Notice that I had to check for the presence of the isBeingEdited property. A customer in the query-results set may already be in cache in which case it already has the isBeingEdited property.

How about the related orders that accompanied the customers in the payload thanks to the “expand” clause? We’ll have to extend them in the callback logic too.

What if we materialize customers and orders by importing them into the manager (see the Export/Import topic). Fortunately, Breeze raises events when entities are imported and we can add event handlers that patch in the isBeingEdited property.

All of this entity patching is turning into a mess. Imagine trying to patch every entity after every query and import, wherever it occurs in the application.

It isn’t just a mess. It feels wrong. The isBeingEdited property should be part of the Customer definition, not something we tack on as an afterthought. We should be able to make it part of the Customer definition … and we can.

Extend the Type

We’ll extend the Customer definition by adding information to the Customer’s EntityType in the client-side MetadataStore. We’ll do this early in the application, before it makes a single call to the backend service.

We can get a MetadataStore from an EntityManager instance [2] like so:

var serviceName = '...'; // route to Web API controller such as 'breeze/todos/'
var manager = new breeze.EntityManager(serviceName);
var store = manager.metadataStore;

Now we have a store variable that holds the canonical MetadataStore for our application. It’s empty at the moment. We’ll fill it in as we go.

Custom constructors

A natural place to extend an entity definition is in its constructor. We generally don’t need a constructor; Breeze can use its own default constructor to make instances of any entity type. But we can define our own custom constructor if we wish and it’s probably a good idea to do so for our isBeingEdited property example. Let’s write that constructor.

function Customer() {
    this.isBeingEdited = false;

We defined our new property as a simple field even though we could be using Knockout. Two-way bindable KO properties must be observable. Don’t worry, Breeze will transform it later into the kind of property that is appropriate to the prevailing model library. If we’re using Knockout, it will become a KO observable.

Notice that we only defined this one extension property. We didn’t mention the other Customer properties because we know that Breeze will supply them later … once it learns about them from server-supplied metadata [3].

Next, we must register our custom constructor with the application’s MetadataStore before the application either queries for customers or creates a new Customer. Any customers created or materialized before we register will not benefit from our new constructor.

Ideally, we register immediately after defining the MetadataStore, e.g.,

// we defined the 'store' above
store.registerEntityTypeCtor('Customer', Customer);

Now we can query for customers:

var query = breeze.EntityQuery.for('Customers');

  .then(function (data) {
      var customers = data.results;
      customers.forEach(function (customer) {
          // assume Knockout
          if (customer.isBeingEdited() === false) {/* ... */}

Because this is the manager’s first service request. Breeze implicitly fetches the metadata from the service and blends it with the type information in the manager’s MetadataStore. The queried customer data are materialized as Customer entities each with the full complement of mapped data properties (e.g, Name) and with the custom isBeingEdited property.

Now that the manager’s MetadataStore is fully populated, we are ready to create a Customer in the manner typical of a Breeze application [4].

var customerType = manager.metadataStore.getEntityType('Customer');
var cust = customerType.createEntity();

// it's now a KO property with the default value (false) assigned in the constructor
var isEditing = cust.isBeingEdited();
var name = cust.CompanyName(); // Also fine.

Don’t “new” a custom constructor

We strongly recommend that you do not call an entity constructor function directly. This section explains why. Use one of the two Breeze factory functions instead, either EntityManager.CreateEntity() or EntityType.CreateEntity().

The Customer function is a valid JavaScript constructor; it is perfectly legal to write:

var cust = new Customer();
var isEditing = cust.isBeingEdited(); // assume Knockout

It’s legal … but cust isn’t a full-fledged entity yet. This next line will fail.

var name = cust.CompanyName(); // ERROR! CompanyName is undefined

It’s going to fail at runtime because CompanyName hasn’t been defined yet.

Recall that our constructor only defined the isBeingEdited property. The other Customer properties are unknown at this time.

The cust object won’t be usable until we add it to the EntityManager.

var isEditing = cust.isBeingEdited();
var name = cust.CompanyName(); // CompanyName is now defined.

The act of adding the entity to the manager causes Breeze to

  • Add all properties defined in metadata for this entity
  • Rewrite fields to suit the prevailing model library [3].
  • Add entityAspect and the change tracking that goes with it.
  • Execute the post-construction initializer (if one is defined).

The EntityType.CreateEntity() factory function does all of all of this for you too … without adding the entity instance to the manager. But we like the EntityManager.CreateEntity() approach even better.

We think the new Customer() syntax leads to confusion. Avoid it.

Whither the unmapped property value?

The isBeingEdited property is only a property on the client-side Customer entity. The “Customer” class on the server does not have an isBeingEdited property and the backing database does not have an isBeingEdited column in the “Customer” table either.

Breeze detects this fact. It adds the isBeingEdited property to the Customer metadata as an unmapped property. When saving entity changes, Breeze may transmit the values of unmapped properties to the server (which might be interested in them) but in a way that the server can easily ignore.

If you look in the network traffic for the entity within the saveChanges payload, you may see something like this:

{"Id": 42, "Name": "Acme Corp","__unmapped":{"isBeingEdited":false},...}

Importantly, unmapped property values are serialized when you export entities and they are deserialized when you import them later (perhaps into a different EntityManager).

That’s important for mobile applications which need to preserve the local state of cached entities when the application is deactivated (AKA, “tombstoned”). Your deactivation logic may call manager.exportEntities() and write the serialized entity data to local storage.

If it does, you probably want the serialized Customer to retain the state of unmapped properties and restore that state when you reactivate the application and re-import the Customer. Breeze will do that automatically.

If you do not want Breeze to remember the state of this property, you should not define it in the constructor. You may prefer to define it in a post-construction initializer, a feature described later in this topic.

Add methods to the constructor

You can extend an entity with methods as well as properties. The methods could be instance methods or methods defined on the prototype. Here’s a frivolous example of the more typical prototype method:

function Customer() {/* ... */}

Customer.prototype.sayHi = function () {
    return 'Hi, my name is ' + this.CompanyName();

store.registerEntityTypeCtor('Customer', Customer);

var customerType = store.getEntityType('Customer');
var cust = customerType.createEntity({CompanyName: 'Acme');
var hi = cust.sayHi() // 'Hi, my name is Acme'

The post-construction initializer

If you need to perform some action after an entity has been created or materialized, you can register that action as post-construction initializer.

Suppose we want the isBeingEdited property but we don’t want Breeze to serialize it locally. If we have to restore a deactivated Customer, we want this property to be false regardless of its prior state.

Instead of defining it in the constructor, we define it in an initializer.

var customerInitializer = function(customer) {
    customer.isBeingEdited = ko.observable(false);

store.registerEntityTypeCtor('Customer', null, customerInitializer);

Now we can create a Customer as we did before.

var customerType = store.getEntityType('Customer');
var cust = customerType.createEntity();

// KO data property with value (false) assigned in the constructor
var isEditing = cust.isBeingEdited(); // still works
var name = cust.CompanyName(); // Also fine.

The customer object appears to be the same as it was when we used the constructor. But there is an important difference: the isBeingEdited property is no longer recorded in the metadata as an unmapped property.

In fact, the Customer metadata have no record of this property. Breeze won’t track it, won’t notify you when the property value changes, won’t validate it, and won’t serialize it.

Notice that we coded isBeingEdited as a KO observable. Had we coded it as a field, it would still be a field. Breeze does not re-write properties that you add to an instance with an initializer. A KO bound control would not be updated when its value changed.

We are using Knockout in this example and we want two-way binding for this property. Therefore, we must define it as an observable. It can stay a field if we’re using AngularJS for model binding.

The post-construction initializer is particularly useful when you must respond to materialized values after a query or import. Here’s an example:

function creatureInitializer(creature) {
    if (creature.Name() === 'Godzilla') {
         'Run! Godzilla is here!');

Register both constructor and initializer

You can register both a custom constructor and an initializer at the same time:

function Customer() { ... }
function customerInitializer(customer) { ... }

store.registerEntityTypeCtor('Customer', Customer, customerInitializer);

Entity creation sequence

Suppose you’ve got a custom constructor and a custom initializer function and you create a new entity with an initial values hash object like so:

new customer = manager.createEntity('Customer', { Name: 'Acme' });

What is the order of events? The answer is illustrated in the ‘createEntity sequence …’ test in the entityExtensionTests module of DocCode.

  1. custom constructor
  2. initial values (‘Acme’)
  3. initializer function
  4. add to manager

When Breeze materialize an object through query or entity import, the sequence is

  1. constructor
  2. initializer function
  3. merge into cache

Camel Case properties

The entity property names on the client exactly match the spelling of the corresponding property names on the server. That’s the default Breeze behavior.

You can change it. For example, you can switch to camel casing (“FirstName” on the server becomes “firstName” on the client) in a single line of configuration:


You can create your own conventions as well. Learn more in the NamingConvention topic.

Temporary key generation

When the entity key (typically an ID) is determined on the server we say that it is “store generated.” We won’t know the permanent key value of a new entity until it is saved successfully. Until then, new entity keys must have temporary values.

Not just any values either. All entities in cache must have unique keys. That means the temporary values must be unique within a cache and they cannot collide with “real” key values coming from the server.

Breeze ships with a default temporary key generator, breeze.KeyGenerator. This is perfectly adequate for most models. But occasionally a legacy model comes along with unique key generation requirements.

Fortunately, you can create a custom key generator and register it with an EntityManager before adding entities to that manager. Make sure your generator can create unique keys for any entity type that needs a temporary key value. The basic structure of a key generator is as follows:

manager.setProperties({ keyGeneratorCtor: myKeyGenerator });

function myKeyGenerator() {
    this.generateTempKeyValue = function (entityType) {
        /* logic here */
        return nextId;

The entityExtensionTests module in the DocCode Teaching Tests has a simple example. The Breeze breeze.KeyGenerator is the best source of inspiration.

ECMAScript 5 Defined Properties

Modern browsers - those whose JavaScript engines support ECMAScript 5 (ES5) or later - can define Object properties with getters and setters. These properties look like the “properties” of earlier JavaScript versions.

var bob = new Person();
foo.age = 42;               // an ES5 property
bob.firstName = 'Bob';      // not an ES5 property 
console.log(bob.age);       // 42
console.log(bob.firstName); // 'Bob'

But they have different implementations and different consequences. Here’s the how we might have defined Person.

function Person() {
    this.backingFields = {
        _age: 0   // default
	this.firstName = ''; 
   	    this.lastName  = ''; 

// Define the ES5 property on the prototype
// it stores the value inside the instance's backingFields
Object.defineProperty(Person.prototype, 'age', {
    enumerable:   true,
    configurable: true,
    get: function() {return this.backingFields._age;},
    set: function(value) {
        if (value !== this.backingFields._age) {
            alert("Foo age changed to "+ value);
            this.backingFields._age = value;

Now when you set bob.age == 42 in a browser, you’ll see a message about the change in an alert box.

Defined properties are particularly convenient when you want to build behavior into setters … behavior such as change tracking, change notification, and property validation.

Breeze relies on ES5 defined properties when you choose to build your application with AngularJS or Aurelia and select the “backingStore” model library … for precisely these reasons.

They are also good for calculated properties in AngularJS apps. Here’s a fullName read-only property that AngularJS can watch.

Object.defineProperty(Person.prototype, 'fullName', {
    enumerable:   true,
    configurable: true,
    get: function() {return this.firstName + ' ' +  this.lastName;}
    // no setter

var sally = new Person();
sally.firstName = 'Sally';
sally.lastName = 'Jones';

sally.fullName; // Sally Jones

// user changes 'Sally' to 'Betsy' via textbox
// angular binding reinvokes sally.fullname
sally.fullName; // Betsy Jones

“enumerable” and “configurable”

If you’re writing an AngularJS application and you are extending a Breeze entity constructor with an unmapped ES5 defined property you must set enumerable: true and configurable: true.

You want Breeze to watch these properties. You want Breeze to validate and serialize these properties. To do that, Breeze must discover and wrap them with behavior to perform these tasks. It can’t find them without enumerable: true. It can’t wrap them without configurable: true.

More about defined properties

Learn more about ES5 properties on the web.

These techniques only work in ES5+ browsers. The Object.DefineProperty feature cannot be shimmed (aka, “polyfilled”) into older browsers. Some browsers, such as IE8, appear to support ES5 defined properties but actually don’t.

Knockout computed properties

Knockout computeds are observable functions that return new values when one (or more) of their dependent observable properties change.

Add Knockout computeds to the initializer

The initializer is an ideal place to define them.

Suppose we want to add a fullName computed property to the Northwind Employee type that combines the FirstName and the LastName. The fullName should be recomputed and re-displayed when either the first or last name changes.

Here is a computed fullName property in an initializer:

function employeeInitializer(employee) {
    employee.fullName = ko.computed(function () {
        return employee.FirstName() + ' ' + employee.LastName();

store.registerEntityTypeCtor('Employee', null, employeeInitializer);

Adding Knockout computeds to the constructor

You might have thought to define the fullName in the custom constructor.

We recommend that you don’t. You’ll almost always prefer to add KO computed properties to an initializer instead of the constructor.

Nonetheless, it can be done and we’ll show you how in case you have a good reason to do so.

Your first attempt might be something like this:

function Employee () {
    this.fullName = ko.computed(
        function () {
            return this.FirstName() + ' ' + this.LastName();
        }, this);

store.registerEntityTypeCtor('Employee', Employee);

var employeeType = store.getEntityType('Employee');
var emp= employeeType.createEntity({
    FirstName: 'John',
    LastName: 'Doe'
var name = emp.fullName(); // 'John Doe'

This will not turn out well. The moment you create an Employee you’ll get an error complaining that “the object doesn’t support property or method ‘FirstName’.”

This FirstName property is defined for the Employee EntityType. But the property doesn’t exist yet on the instance returned by the constructor. Check again to confirm that the only “property” defined in the constructor is “fullName”. The new instance has no FirstName property.

The FirstName property won’t exist until the Breeze createEntity method adds it (and LastName and all of the other properties defined in metadata) to the instance.

Sadly, Knockout doesn’t give Breeze time to add these properties. KO executes the fullName function immediately, within the Employee constructor function. Of course it fails.

There is an alternative syntax for defining a computed property that delays Knockout’s invocation of the fullName observable function:

function Employee () {
    this.fullName = ko.computed({
        read: function () {
            return this.FirstName() + ' ' + this.LastName();
        // required because FirstName and LastName not yet defined
        deferEvaluation: true

    }, this);

The deferEvaluation: true tells Knockout to wait.

Defining Knockout mapped properties in the constructor

In our constructor examples we have only defined extended properties and methods. There is rarely reason to define properties that are already described in metadata.

But you can if you want to and maybe we want to do so to simplify the fullName computed and avoid the deferEvaluation: true syntax:

function Employee() {
    this.FirstName = ko.observable(''); // default FirstName
    this.LastName  = ko.observable('');  // default LastName
    this.fullName  = ko.computed(
            function () {
                return this.FirstName() + ' ' + this.LastName();
            }, this);

store.registerEntityTypeCtor('Employee', Employee);

We didn’t define all of the Employee properties, just the two involved in the fullName computed property.

Now Knockout can execute the fullName computed property immediately upon construction because the FirstName and the LastName properties on which it depends are defined in the constructor.

By the way, There is no harm in defining the FirstName and the LastName properties within the constructor. Breeze recognizes that they are actually mapped to data properties on the server. Their values will be sent with service data during materialization and changed values will be sent to the server during a save.


[1] In this topic we assume that we’re getting most of our metadata from the server via a Breeze Web API controller and that the application should create entities suitable for data binding with Knockout (KO). These are the Breeze dataService and modelLibrary configuration defaults. Learn about alternative Breeze configuration elsewhere in this documentation.

[2] Here we acquire the canonical application MetadataStore from a single EntityManager instance. What if our application needs multiple EntityManagers? We’d want them all to share the same MetadataStore. No problem. First we create the common store

var serviceName = '...'; // the service endpoint e.g., 'breeze/todos/'
var store = new breeze.MetadataStore(); // define metadataStore for all managers

The we create an EntityManager factory method to create new instances that share the common store … and our extensions to it.

function createManager() {
    return new breeze.EntityManager({
        serviceName:   serviceName, 
        metadataStore: store

var em1 = createManager();

// ... later in the application ...
// a new, empty manager with the same metadataStore
var em2 = createManager();

There’s another way too. Suppose you’ve created and configured a manager and you want to create another one just like it with the same store and other configuration settings. Use the EntityManager.createEmptyCopy method.

var em1 = new breeze.EntityManager({
            serviceName:   serviceName, 
            metadataStore: store

// ... later in the application ...
// a new, empty manager with the same configuration including the store
var em2 = em1.createEmptyCopy();

[3] You don’t have to mention properties that are already defined in metadata coming from the service. But it’s harmless if you do.

function Customer() {
    this.isBeingEdited = ko.observable(false);
    this.CompanyName = ko.observable('your-default-name');

Here we define the CompanyName which is a data property mapped to the [CompanyName] column of the [Customer] table in the database on the server. We also specified a value for the CompanyName property. This is the default value that Breeze assigns to CompanyName when you create a new Customer instance.

[4] In this example we are taking advantage of the fact that Breeze fetched metadata from the server implicitly when we queried for customers. Most applications query the server for something before they do much of anything else … and any query will trigger metadata retrieval.

However, if your application might consume metadata before the first query - for example if it might create a new Customer - , you should first fetch the metadata explicitly yourself. Here’s an example: