---
lang: en
title: 'Model'
keywords: LoopBack 4.0, LoopBack 4
sidebar: lb4_sidebar
permalink: /doc/en/lb4/Model.html
---

## Overview

A model describes business domain objects, for example, `Customer`, `Address`,
and `Order`. It usually defines a list of properties with name, type, and other
constraints.

{% include note.html content="Models describe the shape of data. Behavior like CRUD operations is added by repositories. This is different from LoopBack 3.x where models implement behavior too." %}

{% include tip.html content="A single model can be used with multiple different Repositories." %}

Models can be used for data exchange on the wire or between different systems.
For example, a JSON object conforming to the `Customer` model definition can be
passed in REST/HTTP payload to create a new customer or stored in a document
database such as MongoDB. Model definitions can also be mapped to other forms,
such as relational database schemas, XML schemas, JSON schemas, OpenAPI schemas,
or gRPC message definitions, and vice versa.

There are two subtly different types of models for domain objects:

- Value Object: A domain object that does not have an identity (ID). Its
  equality is based on the structural value. For example, `Address` can be
  modeled as a `Value Object` because two US addresses are equal if they have
  the same street number, street name, city, and zip code values. For example:

```json
{
  "name": "Address",
  "properties": {
    "streetNum": "string",
    "streetName": "string",
    "city": "string",
    "zipCode": "string"
  }
}
```

- Entity: A domain object that has an identity (ID). Its equality is based on
  the identity. For example, `Customer` can be modeled as an `Entity` because
  each customer has a unique customer id. Two instances of `Customer` with the
  same customer id are equal since they refer to the same customer. For example:

```json
{
  "name": "Customer",
  "properties": {
    "id": "string",
    "lastName": "string",
    "firstName": "string",
    "email": "string",
    "address": "Address"
  }
}
```

Currently, we provide the `@loopback/repository` module, which provides special
decorators for adding metadata to your TypeScript/JavaScript classes in order to
use them with the legacy implementation of the
[datasource juggler](https://github.com/strongloop/loopback-datasource-juggler).

## Definition of a Model

At its core, a model in LoopBack is a simple JavaScript class.

```ts
export class Customer {
  email: string;
  isMember: boolean;
  cart: ShoppingCart;
}
```

Extensibility is a core feature of LoopBack. There are external packages that
add additional features, for example, integration with the juggler bridge or
JSON Schema generation. These features become available to a LoopBack model
through the `@model` and `@property` decorators from the `@loopback/repository`
module.

```ts
import {model, property} from '@loopback/repository';

@model()
export class Customer {
  @property()
  email: string;
  @property()
  isMember: boolean;
  @property()
  cart: ShoppingCart;
}
```

## Model Discovery

LoopBack can automatically create model definitions by discovering the schema of
your database. See [Discovering models](Discovering-models.md) for more details
and a list of connectors supporting model discovery.

## Using the Juggler Bridge

To define a model for use with the juggler bridge, extend your classes from
`Entity` and decorate them with the `@model` and `@property` decorators.

```ts
import {model, property, Entity} from '@loopback/repository';

@model()
export class Product extends Entity {
  @property({
    id: true,
    description: 'The unique identifier for a product',
  })
  id: number;

  @property()
  name: string;

  @property()
  slug: string;

  constructor(data?: Partial<Product>) {
    super(data);
  }
}
```

Models are defined primarily by their TypeScript class. By default, classes
forbid additional properties that are not specified in the type definition. The
persistence layer respects this constraint and configures underlying
PersistedModel classes to enforce `strict` mode.

To create a model that allows both well-defined but also arbitrary extra
properties, you need to disable `strict` mode in model settings through the CLI
and tell TypeScript to allow arbitrary additional properties to be set on model
instances.

```ts
@model({settings: {strict: false}})
class MyFlexibleModel extends Entity {
  @property({id: true})
  id: number;

  // Define well-known properties here

  // Add an indexer property to allow additional data
  [prop: string]: any;
}
```

### Model Decorator

The model decorator can be used without any additional parameters, or can be
passed in a ModelDefinitionSyntax:

<!-- should be replaced with a lb4 example when possible -->
<!-- according to https://github.com/strongloop/loopback-datasource-juggler/blob/master/lib/model-builder.js#L283 and the legacy juggler file-->

```ts
@model({
  name: 'Category',
  settings: {
    // etc...
  },
  // define properties by @property decorator below
})
class Category extends Entity {
  // etc...
  @property({type: 'number'})
  categoryId: number;
}
```

The model decorator already knows the name of your model class, so you can omit
it.

```ts
@model()
class Product extends Entity {
  name: string;
  // other properties...
}
```

However, the model decorator in LoopBack 4 is not exactly the same as what it is
in LoopBack 3. For example, in
[lb3](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html) we can
pass in a model definition syntax in the model decorator, such as properties,
options, relation etc. But not all these entries are available in lb4 model
decorator:

<!-- Please modify this part when options is available -->

NOTICE: in LoopBack 4 we only support `settings` in the ModelDefinitionSyntax
for now. Those `top-level properties` in lb3 now are passed in `settings`.

- `properties` now are defined in `@property` decorator (see below for more
  information).
- [`options`](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#options)
  in lb3 doesn't have the mapping feature in LB4 yet. (see
  [issue #2142](https://github.com/strongloop/loopback-next/issues/2142) for
  further discussion.)

As for entries in `settings`, LoopBack 4 supports these built-in entries for
now:

#### Supported Entries of Settings

<!-- These entries might need to update once we've made some changes:
  - description [in lb3 we support array or string, but here we documented it as string only]
  these two don't seem work in lb4. Moved them to unsupported table.
-->

  <table>
  <thead>
  <tr>
    <th width="160">Property</th>
    <th width="100">Type</th>
    <th>Default</th>
    <th>Description</th>
  </tr>
  </thead>

  <tbody>

  <tr>
  <td><code>description</code></td>
  <td>String</td>
  <td>None</td>
  <td>
    Optional description of the model. We only support string type for now. (see <a href="https://github.com/strongloop/loopback-next/issues/3428">issue #3428</a> for more discussion.)
  </td></tr>

  <tr>
  <td><code>forceId</code></td>
  <td>Boolean</td>
  <td><code>true</code></td>
  <td>
    If true, prevents clients from setting the auto-generated ID value manually.
  </td>
  </tr>

  <tr>
    <td><code>strict</code></td>
    <td>Boolean or String</td>
    <td><code>true</code>.<br/></td>
    <td>
      In LB4, the default for this entry is set to be <code>true</code>.<br/>
      Specifies whether the model accepts only predefined properties or not. One of:
      <ul>
      <li><code>true</code>: Only properties defined in the model are accepted. Use if you want to ensure the model accepts only predefined properties.
      If you try to save a model instance with properties that are not predefined, LoopBack throws a ValidationError.
      </li>
      <li><code>false</code>: The model is an open model and accepts all properties, including ones not predefined in the model.
        This mode is useful to store free-form JSON data to a schema-less database such as MongoDB.
      </li>
      <li><code>"filter"</code>: Only properties defined in the model are accepted.
      If you load or save a model instance with properties that are not predefined, LoopBack will ignore them. This is particularly useful when dealing with old data that you wish to lose without a migration script.
      </li>
      </ul>
    </td>
  </tr>

  <tr>
    <td><code>idInjection</code></td>
    <td>Boolean</td>
    <td><code>true</code></td>
    <td>
      Whether to automatically add an <code>id</code> property to the model:
    <ul>
    <li><code>true</code>: <code>id</code> property is added to the model automatically. This is the default.</li>
    <li><code>false</code>: <code>id</code> property is not added to the model</li>
    </ul>
    See <a href="#id-properties">ID properties</a> for more information.  The <code>idInjection</code> property in <code>options</code> (if present) takes precedence.
    </td>
  </tr>

  <tr>
    <td><code>name</code></td>
    <td>String</td>
    <td>None</td>
    <td>Name of the model.</td>
  </tr>

  <tr>
    <td><code>scopes</code></td>
    <td>Object</td>
    <td>N/A</td>
    <td>See <a href="https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#scopes">Scopes</a> in lb3 docs.</td>
  </tr>

  </tbody>

  </table>

#### Unsupported Entries of Settings

<!-- Please update the OPTIONS, ACLS field when they are available -->

<table>
  <thead>
    <tr>
      <th width="160">Property</th>
      <th>Description</th>
    </tr>
  </thead>

  <tbody>

  <tr>
    <td><code>acls</code></td>
    <td>
      (TBD)
    </td>
  </tr>

  <tr>
  <td><code>base</code></td>
  <td>This entry is no longer being used. This is done by the typical Js/Tsc classes inheritance way in LB4:
  <pre><code>@model() class MySuperModel extends MyBaseModel {...}</code>
  </pre></td>
  </tr>

  <tr>
  <td><code>excludeBaseProperties</code></td>
  <td>(TBD)</td>
  </tr>

  <tr>
  <td><code>http</code></td>
  <td> This entry affects HTTP configuration in LB3. Since in LB4 http configurations are inferred from controller members and the rest server, this field is not applicable.</td>
  </tr>

  <tr>
    <td><code>options</code></td>
    <td>
      (TBD) see <a href="https://github.com/strongloop/loopback-next/issues/2142">issue #2142</a> for further discussion.
    </td>
  </tr>

  <tr>
    <td><code>plural</code></td>
    <td>This entry is part of HTTP configuration in LB3. So it's not applicable for the same reason as <code>http</code> above.</td>
  </tr>

  <tr>
    <td><code>properties</code></td>
    <td>This entry is no longer being used as we introduced <code>@property</code> decorator in LB4. See <code>@property</code> decorator below to discover moer about how to define properties for your models.</td>
  </tr>

  <tr>
    <td><code>relations</code></td>
    <td>
      With the introduction of <a href="https://loopback.io/doc/en/lb4/Repositories.html">repositories</a>, now <code>relations</code> are defined by <code>relations decorators</code> in LB4.
      See <a href="https://loopback.io/doc/en/lb4/Relations.html">Relations</a> for more details.
    </td>
  </tr>
  <tr>
    <td><code>remoting.<br/>normalizeHttpPath</code></td>
    <td>
    This entry is part of HTTP configuration in LB3. So it's not applicable for the same reason as <code>http</code> above.
    </td>
  </tr>

  <tr>
    <td><code>replaceOnPUT</code></td>
    <td>This entry is no longer supported as LB4 controllers scaffolded by LB4 controller, PUT is always calling replaceById. Users are free to change the generated code to call <code>patchById</code> if needed.</td>
  </tr>
  </tbody>
</table>

To discover more about `Model Decorator` in LoopBack 4, please check
[legacy-juggler-bridge file](https://github.com/strongloop/loopback-next/blob/2fa5df67181cdcd23a5dce90c9c640fe75943cb8/packages/repository/src/repositories/legacy-juggler-bridge.ts)
and
[model-builder file](https://github.com/strongloop/loopback-datasource-juggler/blob/master/lib/model-builder.js).

#### Hidden properties

The properties are stored in the database, available in JS/TS code, can be set
via POST/PUT/PATCH requests, but they are removed from response bodies
(`.toJSON()` output).

To hide a property, you can use the `hiddenProperties` setting like this:

```ts
@model({
  settings: {
    hiddenProperties: ['password']
  }
})
class MyUserModel extends Entity {
  @property({id: true})
  id: number;
   @property({type: 'string'})
  email: string;
   @property({type: 'string'})
  password: string;
  ...
}
```

### Property Decorator

<!-- Property decorator can reuse lb3 docs -->

The property decorator takes in the same arguments used in LoopBack 3 for
individual property entries:

```ts
@model()
class Product extends Entity {
  @property({
    name: 'name',
    description: "The product's common name.",
    type: 'string',
  })
  public name: string;

  @property({
    type: 'number',
    id: true,
  })
  id: number;
}
```

The complete list of valid attributes for property definitions can be found in
LoopBack 3's
[Model definition section](https://loopback.io/doc/en/lb3/Model-definition-JSON-file.html#properties).

You can also specify the validation rules in the field `jsonSchema`. For
example:

```ts
@model()
class Product extends Entity {
  @property({
    name: 'name',
    description: "The product's common name.",
    type: 'string',
    // Specify the JSON validation rules here
    jsonSchema: {
      maxLength: 30,
      minLength: 10,
    },
  })
  public name: string;
}
```

Check out the documentation of
[Parsing requests](Parsing-requests.md#request-body) to see how to do it in
details.

<!-- NOTE(kjdelisle): Until we have a metadata docs section, link to the
package in the repository. -->

The property decorator leverages LoopBack's
[metadata package](https://github.com/strongloop/loopback-next/tree/master/packages/metadata)
to determine the type of a particular property.

```ts
@model()
class Product extends Entity {
  @property()
  public name: string; // The type information for this property is String.
}
```

### Array Property Decorator

There is a limitation to the metadata that can be automatically inferred by
LoopBack, due to the nature of arrays in JavaScript. In JavaScript, arrays do
not possess any information about the types of their members. By traversing an
array, you can inspect the members of an array to determine if they are of a
primitive type (string, number, array, boolean), object or function, but this
does not tell you anything about what the value would be if it were an object or
function.

For consistency, we require the use of the `@property.array` decorator, which
adds the appropriate metadata for type inference of your array properties.

```ts
@model()
class Order extends Entity {
  @property.array(Product)
  items: Product[];
}

@model()
class Thread extends Entity {
  // Note that we still require it, even for primitive types!
  @property.array(String)
  posts: string[];
}
```

Additionally, the `@property.array` decorator can still take an optional second
parameter to define or override metadata in the same fashion as the `@property`
decorator.

```ts
@model()
class Customer extends Entity {
  @property.array(String, {
    name: 'names',
    required: true,
  })
  aliases: string[];
}
```

### JSON Schema inference

Use the `@loopback/repository-json-schema module` to build a JSON schema from a
decorated model. Type information is inferred from the `@model` and `@property`
decorators. The `@loopback/repository-json-schema` module contains the
`getJsonSchema` function to access the metadata stored by the decorators to
build a matching JSON schema of your model.

```ts
import {model, property} from '@loopback/repository';
import {getJsonSchema} from '@loopback/repository-json-schema';

@model()
class Category {
  @property()
  name: string;
}

@model()
class Product {
  @property({required: true})
  name: string;
  @property()
  type: Category;
}

const jsonSchema = getJsonSchema(Product);
```

`jsonSchema` from above would return:

```json
{
  "title": "Product",
  "properties": {
    "name": {
      "type": "string"
    },
    "type": {
      "$ref": "#/definitions/Category"
    }
  },
  "definitions": {
    "Category": {
      "properties": {
        "name": {
          "type": "string"
        }
      }
    }
  },
  "required": ["name"]
}
```

If a custom type is specified for a decorated property in a model definition,
then a reference
[`$ref`](http://json-schema.org/latest/json-schema-core.html#rfc.section.8)
field is created for it and a `definitions` sub-schema is created at the
top-level of the schema. The `definitions` sub-schema is populated with the type
definition by recursively calling `getJsonSchema` to build its properties. This
allows for complex and nested custom type definition building. The example above
illustrates this point by having the custom type `Category` as a property of our
`Product` model definition.

#### Supported JSON keywords

{% include note.html content="
This feature is still a work in progress and is incomplete.
" %}

Following are the supported keywords that can be explicitly passed into the
decorators to better tailor towards the JSON Schema being produced:

| Keywords    | Decorator   | Type    | Default      | Description                                             |
| ----------- | ----------- | ------- | ------------ | ------------------------------------------------------- |
| title       | `@model`    | string  | _model name_ | Name of the model                                       |
| description | `@model`    | string  |              | Description of the model                                |
| array       | `@property` | boolean |              | Used to specify whether the property is an array or not |
| required    | `@property` | boolean |              | Used to specify whether the property is required or not |

## Other ORMs

You might decide to use an alternative ORM/ODM in your LoopBack application.
LoopBack 4 no longer expects you to provide your data in its own custom Model
format for routing purposes, which means you are free to alter your classes to
suit these ORMs/ODMs.

However, this also means that the provided schema decorators will serve no
purpose for these ORMs/ODMs. Some of these frameworks may also provide
decorators with conflicting names (e.g. another `@model` decorator), which might
warrant avoiding the provided juggler decorators.
